If you have the file number of the criminal case, you can go to the court and get a "certificate of disposition" (bring photo ID).You will be able to find attorney's name from court. The issue is even your case dismissed, you should indicate any "arrrest" on I-485. If your I-485 was prepared by attorney, better get an affidavit from that attorney. At the end of the day, i think you would be fine.