Well, it's one of those, "you know it when you see it". I'm sure they make up less than 10% of the membership.
Detailed descriptions of what identifies a paid Russian troll would probably exceed the 5K word limit, so I'm also mostly going with the Justice Potter theme of "You know it when you see it."
The one thing that seems to stand out, however, is the distinct lack of local color in their posts.
By that I mean, since this is primarily a site aimed at North American politics, with an emphasis on U.S. politics more than anything else, it's interesting what happens when you press one of these people for some local color.
Example: Here in Whittier, these neighborhoods went from being borderline street gang "no go zones" in the 80's and early 90's back to being the quiet and somewhat boring baby factories that they were in the postwar years, because so many of the gangs moved to the Inland Empire and to Palmdale.
They were, quite simply, priced out of the market and the rest were reduced by attrition due to law enforcement stepping up and doing their job.
Remarks about local cultural and ethnic trends, local housing prices, local employment prospects, food, music, local crime, local "anything-at-all", are notably absent in posts that come from paid Russian trolls, because the only thing local that they could comment on would be 55 Savushkina Street, Saint Petersburg.
You'll also never hear from them about family, kids, job, pets or even what they ate that day.
I don't think any of them have an opinion on issues like pot legalization either, or if they did, would they be pro or con, and why.
It would be interesting to push them on an issue like that, because if they're in Russia, it would be difficult for them to be very aware of local trends, like say for instance, the new pot dispensaries opening up OR being forced to close in "their Amerikansky city".
NOTE: Some of the more sophisticated Russian trolls MIGHT have been to or have friends living IN one of the Russian diaspora cities here like West Hollywood, (Fairfax District) for instance.
One way to trip them up would be to ask about a local business, say perhaps a local eatery which has been closed in the last year. If they say they just came back from lunch there....
BUSTED.
It's maybe a little bit difficult to fake being an American resident if you've never lived here.
You can maybe get away with it for a little while but sooner or later someone can trip you up.
The building in the background must be where you post all of your propaganda. 
