Call the court, give the clerk your name and the case number (if you have it), and politely ask if your conviction has been expunged.  That is all there is to it.  

I once went to a court to ask how to have a family member's misdemeanor conviction expunged.  They said it is easy, just fill out this "expunge request" form, get it notarized, and bring it back.  I sent the form to my family member who was out of state, he filled it out and mailed it back to me, I personally brought the form back to the court clerk, the clerk later sent the form to the judge, the judge asked the prosecutor if he objects, the prosecutor had no objection, so the judge ordered the conviction to be expunged.  This all happened in a matter of 2 days or something.  Frankly, this was easier than getting anything done at the DMV.