No meter needed. Just use your ears. If you hear a large volume change when the subs kick in, they're probably set too high. I suppose you could do it with a meter but your ears should work just fine.

Your just looking to get the smoothest sweep possible from end to end. Unless you have your room tuned, however, you're going to hear some bumps. Just try to make it as smooth as your current system allows.

The other "by ear" bass check for subwoofers is playing a CD containing either acoustic piano or acoustic bass. Listen closely to the bass lines and see if you can hear your crossover point as the bass line goes from higher to lower. Again, if the change to your subwoofers is too much, take a little level off of them. This also relates to what Gonk mentioned where his bass levels are different for movies and music. You're probably going to want more "boom" in your movies than in your music. It's really all about what makes you smile.