Cracks and scratches. That's what to look for.
Brands like Ridley or Scott ... I haven't heard of many fakes ... but do your research. People can put any sticker on any frame.
I would be wary of buying used CF, unless it was in very good condition---I mean the whole bike. If there are scrapes on the brifters but the paint is perfect ....
On the other hand, people who buy CF still like to buy bikes ... just like their old Al ro steel frames hadn't worn out when they sold them to get the next better thing, people who have had their first or second CF bike for a few years are probably getting ready for the next new one (17 grams lighter!!! and just $1000 more!!!)
The coin-tap technique sounds like a goo d place to start. If the bike doesn't look to have been abused, i wouldn't worry ... but I would do regular checks just to see that nothing seemed to be developing.
Or rather I would do regular checks until I finally became convinced the bike was sound ... which would be when a crack would suddenly develop and I wouldn't see it until it got big.
But that's just me.
Seriously, common sense tells me that there will be an increasing number of perfectly good CF frames hitting the market as riders feed their upgrade bugs. Just do an even more thorough inspection than usual and reject any bike or buyer that gives you the least bit of a bad vibe.
Hopefully wiser posters will respond.