You could simply try doing some isolated quads / hip flexor strength tests in the gym. Do some seated quads extensions (single leg) on a cable resistance machine and compare sides in terms of weight / reps. Sit on the edge of a bench with a 10kg weight on your knee (can hold it just for balance), try and lift your knee (while keeping it bent at 90degrees) off the bench and hold for as long as possible. Mark the difference between sides and post them here.

Hyporeflexia (lack of reflexes on the affected side) can also be prevalent and give us a good idea what is going on. Get someone to do a patellar (quads tendon) reflex test, the one you see Dr's on movies etc. doing all the time. Is there a difference between sides? Repeat the test multiple times to ensure it is not just a lack of testing method leading to false results.

If you have hyporeflexia and a large strength deficiet, (you can also do some co-ordination testing under fatigue etc. but the above tests should give us the needed info) then it is likely you have some neural implications (lower motor neuron). If this is +ve an Wikipedia reference-linkMRI on your Lumbar segments would be helpful, along with some further neural testing. Do you have any other symptoms? any strange feelings in the gluts / lower back / hip???