I've struggled with the question of "how intense is INTENSE?". I'm trying to increase the intensity of my workouts and build strength.

Some days I feel really intense, other days I just feel like I'm just going through the motions. I try to measure myself by, oddly enough, sweat. When I feel like I've had a good workout, I'm pretty well spent and my sweat shows. Other workouts, I'm hardly even warm, much less sweating.

So, are there other ways to measure intensity or otherwise know that you're getting a solid workout?