I have been out the gym for 2 months now and plan on going back and doing 5/3/1 again. I was just curious as to how do you know if you are lifting enough? I know the 5.3.1 part because you do the last set to failure, but what about the supplemental lifts? Are you suppose to do those to failure also?

The last set doesn't necessarily have to be done to failure.  Actually in my opinion it's better to leave a rep in the tank.Now the supplemental lifts depend on the exercise whether or not it should be done to failure.  In my opinion an exercise like a front squat or an incline bench press (variations of the main lifts) should not be done to failure.  Now curls, tricep extensions, and leg curls?  Yea you can take those to failure.