what fails first with too much nitrous

[rppngears]

zepherman,
As long as you take car of you stang, change the oil on time and such, 100,000 miles realy isn't that much. The first thing that starts to go, besides if something just breaks because of manufacturers defect, weak spot in metal or misuse (such as reving to 6000 rpms every shift), is the piston rings. With the other possibilities I just mentioned, there is no way to tell about these, but 302's are good engines and usually I don't think you have to worry about those so much. By putting nitrous on an engine with already weakened rings, you can easily finish them off. Especially if you hit the nitrous while you fuel mixture is to lean, or you are not full throttle. But, as long as you don't use anything bigger than a 50 - 100 shot (in my opinion) and don't go through 5 bottles a week, you should be fine.
If you want to check the condition of your rings, besides of course by taking your engine apart, you can perform a compression check. If your compression is what is should be, than your rings are probably ok.