There is the "Sit and Reach" test. Remember this from school?
From taking yoga I think there is way more to flexibility that this though. I am incredibly flexible in the hip flexors. So I have a lot of flexibility when I do a pose like this:
But I have very tight hamstrings, so when I do a pose like this, I have virtually no flexibility:
Studies show that much of your flexibility is genetic but the good news is that you can improve upon it.