Here's a quick test to help you determine if your hamstrings are tight:
While standing, slowly shift your weight back to your heels, and push your hips back as far as possible. Stop immediately when your lower back starts to round. This is the full length of your hamstrings. You want the angle between your thighs and torso to be about 90 degrees. If you're not able to bend at least this far, your hamstrings probably need some work.
But if you don't reach that magic number, it doesn't necessarily mean your hamstrings are too short. They might be the right length, but pulled tight because of a pelvis that tilts forward. In this case, the hamstrings are actually trying to restrain the pelvis from tilting farther forward; the result is the tightness you feel in the back of your legs.
You need to know which best describes your situation before you go nuts and start stretching your hamstrings like Gumby on speed. If the problem is anterior pelvic tilt, you could make the problem worse by lengthening your hamstrings. That would allow your pelvis to tilt farther forward.
To rectify this problem, you need to first address the position of your pelvis, which probably has more to do with tightness in your hip flexors than your hamstrings. That means stretching your hip flexors. Once that's accomplished, your hamstring tightness should improve without you having to lengthen the hamstrings themselves.