How do you tell if ignition coil is bad on a 1985 300zx?

Easy way is to run the large out wire to a ground, one post on the coil to a ground and one to positive. Do not allow the large wire to actually touch the ground, have it just above the ground so that you can see a spark.

Warning: do not touch the large wire while the other are hot. It has a lot votage and could possibly cause you to get injured.