You can gauge it with leg extensions & curls, but a more specific test could be with a deadlift or squat maximum, which uses a significant portion of your lower body as a complete chain of movement.

The general strength recommendations between your quads & hams on leg ext/curl strength is that your hams should be 75-80% or greater of the strength of your quads. So, if your quads lift 100 pounds, your hams should lift 75 to 80 or more, for example. If you do a strength test on the leg curls or extensions, might I recommend testing each leg individually? I do this with my athletes and get a better reading of strength imbalances between not only muscle groups, but between legs as well.

If you have a multi-hip machine to do hip flexion and extension exercises on, that would be helpful for you, too, in seeing where your core strength is, in relation to your lower body movement, much moreso than an ab crunch/back extension comparison.

Rick Karboviak, CSCS
http://trinitytraining.bravehost.com