Where would I go to find out?
I would think that the person you or Matt were in contact with at NCMEC would be able to provide that information. Of course I am assuming it was NCMEC who told you your mothers DNA had already been compared to Sharons. Was it NCMEC?
I would start there and if there are roadblocks seek the information other ways. Maybe the freedom of information act? But I am not fully up to speed on FOIA laws and info.
I would really like for your piece of mind, for you to be certain your mothers DNA was in fact tested or not if you believe perhaps it may not have been. Especially since it was several years ago Rhonda was first suggested as a possible match to Sharon and you were not fully involved at the time IIRC.
hth