So how can you tell if you are lifting heavy enough?
Jamie Eason -- this girl definitely trains hard
If you are lifting heavy enough, you should get to the end of your rest period (45-90 seconds, see my training page here) and feel like it wasn't enough time.
If you are lifting heavy enough, you won't have the energy or even the ability to look at other people during sets, much less be able to tell whether or not they are watching you (I'm looking at YOU, people who don't work out because you're self-conscious!)
Don't stop performing an exercise when the reps start to get more difficult to perform. Continue pressing, curling, etc until you literally hit the wall -- as in, you physically cannot perform the motion again. Note that I said cannot, not don't want to. You may be surprised at how many more reps you can get out -- and how much more weight you should, therefore, be using.
Have you ever surprised yourself with your own strength? What has been your biggest aha! moment with exercise?