To determine your intensity level, use your own rating of perceived exertion.
OK, if you can cary on a conversation or sing while working out...
your intensity level is: Low
If you can talk, but not sing while working out, your level is: Moderate
If you can briefly talk, and can't sing while working out, that's a High
level of intensity.
My trainer was nice enough to help me figure out my exercise intensity level. She told me yesterday that I should peddle harder... or learn to lip sync. (Some people just don't appreciate Country Music.)