If you have received a form 5498-SA from your HSA administrator, the FMV (fair market value) of your HSA on December 31, 2017 will be found in box 5.
However, since the HSA administrator is not required to send you the 5498-SA until the end of May, you may have not received it yet.
I suggest that you go to the HSA administrator's website and look at your account there - many HSA administrators or custodian put this kind of information on the Web.
In the worst case, just call them.