One way you can tell a difference between a clone and the real deal is to simply search before you buy. Take time to search - I do for any product I spent my hard-earned cash on. Look at what FastTech has to offer and then Google the same product. If, as stated above, one is selling for $200 and the on at FT is selling for $20, that's a dead giveaway. This doesn't mean clones are bad, but don't expect the world for nothing - you won't get it.
That being said, plenty of B&M's sell clones and I'm more than sure they get them from FT. They also take those same mods and mark them up by $20-$50 each (I've seen it), which is sort of ridiculous if you ask me. I sort of lose all respect for a dealer that does this. I don't mind them selling clones, I encourage it.
I picked up an IGO clone a few weeks back and it's working perfectly, but it was maybe $1-$2 above what FT sold it at, so no big deal to me. For those new to vaping and in areas where vape shops are not side-by-side or down the street, a clone can get someone who doesn't have a lot of cash into vaping more and more, more so than an eGO can or would. Everyone I know eventually moves away from eGO's and on to bigger and better things and clones assist with that in a way by offering an affordable alternative.
When the time comes to get something better, new-to-vaping users can use the experience with the clone to find what they like and dislike and apply that to the next mod they get.