I have no clue on art etc but didn't like to see an unanswered question!
If it was me I think I'd ask 2 reputable gallery's if they can recommend anyone - and then get 2 opinions.
If you don't have any near you you could even phone around you may not even need to take the painting anywhere for a rough idea - if they know their stuff and you tell them the name and condition it's in then they should be able to give you a price range over the phone I would have thought.