As a rule if they get a bit warm that won't kill them, it will just reduce the lifespan of the culture a bit. Normally you would use the batch up before it would run "out of puff".
Almost all of my cultures are delivered by post, and I have only had one fail before I used them up.
To directly answer your question, just warm a cup of milk to blood temperature, add a tiny grain of culture and stir. Wait an hour and add a drop of rennet, it will eventually set if the culture is good.
Basically make a tiny test batch of curds. Even easier if you have a Ph meter, just add culture and check later to see if the Ph has dropped.
