If I filed for divorce in 1999 but never received a certificate, where can I check if I'm legally divorced?

Check with the Family Court in which your case was filed. If you are divorced there will be in your file there a Judgment terminating marital status and a Notice of Entry of the Judgment.

Most Courts have a website where you can look up the case using the case number. If you don't have a case number then a search for the case itself would have to be conducted using the parties' names.

There is quite a bit that you (or your spouse) is required to do after filing your request for dissolution of marriage. If you didn't take care of all of it and you didn't get anything in the mail saying your spouse was requesting to finalize it and the Judgment was entered, then it is very unlikely you are divorced. You need to check at the courthouse where you filed your paperwork. Hopefully, you still have your case number, but if you don't, you will be able to look it up and then see if a Judgment was ever filed. If a Judgment was filed, you will want to obtain a copy of it, it has the orders you were/are required to follow and it is the proof you need to show you are divorced.