Before we even get into testing flexibility, the question is: does it matter? The simple answer is yes, but as I'll explain it depends on what you need that flexibility for. To test it:
A good level of flexibility is hitting around 90 degrees straight in the air. But I find it alright if someone gets to 80 degrees depending on their sport and activity.
But when I do this test what I really look for is on the opposite die. When the leg goes up , what's the pelvis doing? If there's a lot of tilting and bending happening, this is more my concern. It means in a functional task like a soccer kick or sprint you're probably compensating and don't have TRUE flexibility of that hamstring.
For movement geeks it means you have poor extension or tight hip flexors on the opposite side. It can also mean poor hip mobility on the same side.
Hamstring 'inflexibility' is related to injury (Clarke, 2008). But it depends on WHY inflexibility is happening. You can't just stretch your hamstrings forever, you have to address core stability issues, other muscle balances as well.
Do this simple test yourself, do you have good active hamstring flexibility?
Milner Chiropractic and Sports Injury Clinic