From my experience, I can most DEFINITELY tell the difference. I've had at least two pairs of fake TR's in my hand in the past. One was super stiff and scratchy (although this wasn't noticeable until I had a pair of real ones to compare, and I was new to the whole jeans thing) and the other felt softer, but thin almost like it wasn't denim...if that makes sense. I agree with Pam when she says:
Fakes feel nasty. $2 denim feels like $2 denim, $200 denim feels like $200 denim. You get what you pay for.