Last weekend I was re-assembling my engine, and three holes stripped, is there any way to see if a screw hole needs to be re-threaded before it strips?
Clean the hole up as best you can and get lots of light on the scene. Try screwing a bolt into it and seeing if it bites and tightens up properly (obviously one with the right thread pitch). If it won't bite or won't tighten, it's time to break out the tap and die set.
"Stripped" means the threads are damaged. Re-threading (using a tap) cuts new threads, and cutting new threads out of damaged threads in a hole that is probably too big will not make it any better.
I'm guessing your engine is aluminum? Bolt holes strip very easily in aluminum. If the bolt doesn't go in easily you are doing something wrong. If you overtightened the bolt the threads are likely damaged beyond easy repair. Did you use a torque wrench? If no, you have very likely over-tightened the bolt.
You want something like a helicoil thread repair kit, you can get one at any decent tool shop.
Reply to comment: 8Nm is at the low end of the spark plug / oil bolt range. If it is an oil bolt or similar non-structural part you can usually drill it out to the next larger size