Where can I find an expert (or committee) that can review images of a painting that I own to determine if it could be one of Sargent's "lost" paintings that he created somewhere in the Middle East in the late 19th or early 20th century? I wish to do this very discreetly because of the potential value of the work. Thank you.

If you want to do it 'discretely', why did you ask on a very public forum ?

For identification of the painting, it would be best if someone who is an expert could examine it in person rather than by viewing images. I would suggest taking it to an art gallery or an art professor at a college or university. You might even have some success by consulting an auctioneer.