You would have to know your past payment history. Possibly you can get an account transcript from the IRS if you did not save copies of your tax return.
https://www.irs.gov/individuals/get-transcript
You can also look up your FTHB account here.
https://www.irs.gov/credits-deductions/individuals/first-time-homebuyer-credit-account-look-up
In general, repayments of the 2008 credit would have started on the 2010 tax return, so after filing the 2019 return you would have paid 10 x $500, and owed just $2500. The IRS may have failed to credit one of your payments, but you will have to do some research to prove it.
