Fake tends to stay a clear liquid in a pipe no matter how much heat is applied (gear tends to go yellowy/brown), it's really watery (gear tends to move around the pipe slowly and oily), it has no or weak taste (gear tends to have a strong taste and odour), you don't blow out much, if anything (gear tends to make you blow out clouds), it crushes into a white powder straight away in the bag when you try to crush the chunks (real gear rocks tend to be a lot harder and makes louds cracks as it breaks down into smaller, drier chunks)... it sucks hey and it looks so convincing upon first glance. Fake rocks are also whiter and smoother, real rocks are clearer and have jagged edges.
Crush a chunk and if it goes really white and powdery, might be a good idea to stay away.