I have been in this situaion a few times. somehow cannot resist jacking with people who don't comply with the regulations.

There are poacher hotline numbers listed on signs in several locations in the areas I have been fishing here in CA. Your state may have them too. look for them to be listed in the regulations booklet.

I go out of my way to get close and introduce myself in a friendly manner. If the poacher is displaying a liscence, you can take the name and or liscence number and report him/her without even getting confrontational. The last time I smelled a rat I caught a couple (husband and wife) with a stringer full of really small fish. They were bragging about how easy they were to catch (they were using live hoppers). I asked them to hold up the stringer so I could get a picture. In 5 minutes I collected their names, fishing liscence number (she didn't have one, but caught several) and photographed them with poached wildlife. BUSTED!

Another time I was fishing and a guy was trying to catch trout with a string, a soda bottle (bobber) and worms (no liscence). The game warden approched him and asked him if he was fishing and the guy pretended he knew no English. However, I ratted him out from accross the river and let the warden know that I conversed with him in English.
BUSTED!