It will usually smell awful if it has gone bad.

if the mice died and you froze them right after they died, then they should be perfectly fine to feed to your snake. The mouse SHOULD have pink ears feet and tail...at least, that is what a normal live mouse looks like.

Just make sure you heat the rodent up well before you offer it so the snake can recognize it as a food source. If it is too cool then it will be hard for the snake to get a good strike. I use a hair dryer on the heads of my rats right before I feed them. Just a couple seconds is really all you need