Short answer: With your eyes.

Okay now that that is out of the way, what techniques do you use to observe your RETFs without disturbing their natural behavior. It's a little tricky since these guys are nocturnal and love the pitch black environment. What way works for you? Do you use a flashlight, a light with a dimmer? How do you keep a fairly dark room so they are comfortable yet introduce enough light for you to watch them?