Buy a multimeter. Even the cheapest multimeter you can find works, as every multimeter can measure DC voltage with an accuracy that is enough to say "there is voltage" vs "there is no voltage".
Measure voltage between the positive battery terminal and the point where you connected the negative. Do that in the same configuration you jump started -- if the ignition key was on, let it be on during this test.
If it says "0 volts" (or any voltage close enough to be just random noise), you didn't connect the negative. If this is the case, it's possible the battery just needed rest. Lead-acid batteries are slow: they best work at 20 hours discharge. If you take a lot of current, let's say at 5 minute discharge rate, it may exhaust the battery. Just waiting can let the battery provide good current again. So it's possible the rest of the battery is what allowed starting.
If it says "12 volts" or anything close to that (9 - 15 volts), you did in fact jump start.
There's also another possibility, that there was some fuse on the negative side (unlikely, usually they are in the positive side), or something that worked like a fuse, like a very thin electrical wire. In this case it's possible you did have a connection when jump starting but the connecting wire has already melted and is no longer a wire. You can diagnose this only by looking for the wire, to see if there is a wire that no longer functions like a wire, connected to the point that you used as negative for jump starting.
Jump starting with only one wire is not possible. If it worked, you had two wires or didn't need a jump (battery rest was enough). The other wire could however be many things, like unpainted parts of car metal bodies touching each other.
Next time you jump start, connect to some piece of metal that has strong grounding, like the engine.
If the feeder car has electrical problems or lost seat / radio memory, it's very possible that you did in fact do a jump start.
Also, starting from this point on, if you expect the flat battery needs not just charging but also replacing, don't jump start! A dead battery can't act as a buffer for the car electrical systems. The alternator with a dead battery could cause voltage spikes that create expensive electrical trouble, even so expensive that it may be cheapest to just have the car recycled and buy a new car to replace it. Only jump start if you let the lights on and discovered it in few days at most. Few weeks with a totally flat battery may mean it's already dead and can't be salvaged => don't jump start then!
I made a big mistake today attempting to jump start a car but connected negative clamp (-) to wrong part of the car. Basically there was no negative connection between the two cars. I did not notice until later because the jump start actually did work, and the car with the dead battery started to run again and we got the battery replaced. But I noticed small electrical problems on both cars after the jump start.
