You'll feel the rear wheel start to lift before you get to the point of doing an endo.

Biggest thing though is to keep your arms tense. With good brake pads, your front brake will stop you very quickly and you will really feel pushed forward into the bars. I think a lot of brake endos are caused by someone grabbing a "handful of brake" but not keeping their arms tense. They then flop forward onto/over the bars (think of flopping forward in a car if the driver brakes unexpectedly) and upset the balance of the bike and over you go! If you tense and straighten your arms to keep your weight rearward you'll be fine. On a bike with a freewheel, I'll hang my butt off the back of the saddle even.