Reckon you could check the pcgs or ngc cert and if it was reported stolen then they might have something on the cert page
Can a person report these "as stolen" to the TPG itself? Or do the TPGs not want to get involved in such things? I don't think there is any such mechanism now. I can see the downsides to TPGs handling "stolen" reports: You could sell a coin to someone, report it stolen, then when the current owner tries to sell it, there's an alert that comes up that says it's stolen. Yeah. I could see why TPGs wouldn't want to get involved...
This site features a link to coin week article (a rather unnecessarily rambling one at that), "Anatomy of a Coin Thief". We better keep our eyes out for this guy, he's pretty good at stealing from shows.