Stomach pumping

Don't know anything about the correct way, so this question might seem obvious to most. When you pump the stomach on a trout, is that only done on a dead one? Does the act of putting the pump down their throat do too much damage to release? I've never pumped the stomach of a trout, is there a method to do this correctly without causing damage to the fish? Often I'll catch a single trout and not be able to get the interest of others rising and it would be great to get an idea as to what they're eating without causing damage to the fish.
Thanks