Finding information on the service required: call a dealer and ask them. Look online at forums. Etc.
How to tell if it's been neglected, that's the more important part of your question IMO.
You can tell a lot from the car physically, more so in my experience than from service records. Service records are only as useful as the record keeper, and in my experience, more often than not they just don't exist. You'll preclude a lot of good cars (and could possibly be misled into a bad car) by relying service records alone. A lot of good, well-maintained cars might not have paperwork - some owners just don't keep it. Also, it's easy to "loose" records too, and presenting just the ones that make the car look good can be misleading. (What do you mean I forgot to include the records for the three separate head gasket replacement jobs?...I mean no, I never had a problem with head gaskets before...)
If you learn to look the car over physically, you'll be able to tell pretty quickly whether or not the car was well generally well maintained or neglected.
Most cars are going to require the same basic service at similar intervals. Oil, air filters, spark plugs, timing belt, ball joints, wheel bearings, brakes, etc. Specialty vehicles, rare or exotic vehicles, etc. may have different needs - see if you can find a forum with specifics on what to look for in those cases. But generally you can tell pretty quickly whether or not it's been well maintained just by looking at the following:
- Check fluid levels: brake fluid, oil, transmission fluid, power steering fluid, coolant, etc - are they all at the proper level? If no, that's a bad sign - most shops top fluids up whenever you bring in for service, and DIY owners should be on top of this stuff as well. Being one of the simplest things to do, if fluids are not filled to the right level, that's generally a bad sign.
- Check dip-sticks: wipe on a clean white cloth or paper towel and look at color. Motor oil should be gold to brown in color, transmission fluid should be red to brown (neither should be jet black!). They should be clean and translucent, and there shouldn't be metal particles in either. Smell it!! A burnt smell or smell of gasoline or antifreeze could indicate internal engine problems. If these fluids are clean, smell okay, and are at the proper level, that's a good sign.
- Coolant: comes in a variety of colors, from green to orange to red. It should be translucent, and have a sweetish aroma. There should not be any particles. If you see metallic particles or thick sludgy stuff that's a bad sign - probably had a leak and some previous owner used a stop-leak product. That stuff is bad news and just masks a bigger problem.
- Brakes: look at the rotors and feel them by running up/down them with a fingernail - there shouldn't be heavy scoring or deep gouges. Look at the pads (as best you can) - they should have 2-3mm or more material thickness and should be worn evenly (e.g. Not heavily tapered in one direction or another). Surface rust is common, especially if the car is not being driven daily, so don't worry about rust too much.
- Air filters: a big indicator for me usually. Many dealers and curb-stoners are smart enough to change oil and tranni fluid if its dirty, but many overlook these elements. Look at the engine air filter; if its filthy and totally clogged, that's a bad sign. Some dirt is normal - the filters job is to trap dirt - but if it looks like an old dirty bird's nest and has a thick layer of dirt like a 1/4" thick - that's a very bad sign. If you can get to it easily, and the owner will let you, check the cabin air filter. This is a regular maintenance item, but one often overlooked, and this can say a lot about general maintenance of the vehicle. Check YouTube for how to change cabin air filter for the make/model/year of the car and you should find a video on how to replace it - look that up before you go check the car out so you know how to get to it.
- leaks: look for leaks. Power steering hoses, valve covers, head gaskets, etc. are common places for leaks. After test driving, park the car on clean concrete and do a thorough walk around. Take 10-15 minutes to look the car over in and out, and during this time look for any leaks and any new spots forming on the concrete after sitting for 10 minutes or so.
- tires: bring an air pressure gage - are the tires all inflated properly? Are they at similar pressures? Are they all matching brands with similar wear? Is the wear pattern fairly consistent, or is it concentrated on one area of the tires. Proper tire maintenance including air pressure and rotation is a good indicator of general maintenance habits.
- timing belt sticker: on higher mileage cars (e.g. 100k or more miles) they should have had the timing belt changed (if equipped - timing chains are different story, you can google this info for a car). This is typically done by a shop and there should be a sticker somewhere in the engine bay that has date and mileage it was done at. If there is no sticker, ask the owner about it. On many cars it's not a hard DIY thing to do, so if the owner says he did it personally, ask a few questions and try feel out whether he knows what he's talking about or not and if he's telling the truth.
- serpentine belts: AKA accessory belt. These are the visible belts that run around all the pumps and doodads in the front of the engine. Look at them. Touch them. They should be clean and black in color, they should have some flex if you try to twist them about their own axis, and they should not have any cracks or missing teeth. If they are old, discolored and dry-rotted, or have many cracks or missing teeth, that is a bad sign.
- joints: ball joints, cv's, tie-rod ends, steering knuckles, etc. anywhere you see a zirk fitting! Is there grease all over these things, or are they dry as a bone? Nobody likes a dry socket - especially not your car. Greasy joints are happy joints and a dry joint is usually an indicator of poor general maintenance.
Always test-drive the car, and feel it out. Does it run smooth and ride nice? Do you notice any problems and strange sounds or vibrations? If so, ask the owner about them and how long they've been like that and why they haven't been fixed prior.
Lastly, be prepared to walk away. Always.
I remember going to look at a car and when I looked at the brake fluid reservoir it was empty. Like totally empty. Bone dry. And for how long? Who knows. So I looked under the car and found a very obvious slow brake fluid leak almost immediately. I let the owner know about it; he was a younger guy and after informing him and talking to him for five minutes I could tell he was totally clueless about maintenance. It was obvious he didn't know anything about the car and equally obvious he wasn't taking the car in for service, yet he said he had owned it for many years, meaning it was probably pretty well neglected. I walked away from that one, but it was a little scary; had I not told him about the leak, who knows how long he would have kept driving it obliviously, until one day he went to hit the brakes and didn't have any!!
Disclaimer: this answer was an attempt to provide some guidance on what to look for in determining a vehicle's general maintenance history without records, not an attempt to document everything to look for or how to perform a thorough mechanical inspection. Service records can be nice to have, but papers are just papers - they are nothing more. The car itself will usually tell you everything you need to know.
