You know you are getting the most out of your workout when...
You are working up a sweat. No, not every "workout" needs to include an intense sweat, and certain activities it may seem like you're not sweating (swimming or a run on a cold day), but a good sweat is a great sign that you are pushing yourself.
Your muscles are burning!! This is a sign that you are breaking down muscle tissue that will be repaired bigger and stronger!
You sleep like a baby....this is a great sign that you pushed your body and now you are getting great rest and repair sleep.