Since Bitcoin is a currency that was designed to change hands often, there is always a chance of receiving stolen Bitcoins. That shouldn't be a problem though, provided that you didn't steal them yourself. Bitcoin is money in digital form -- that's all it is. As for "fake Bitcoins," that's not even a real thing. An unconfirmed transaction doesn't make a Bitcoin fake. Just wait until the transaction is confirmed, and barring the possibility of a double spend attack resulting from a forked blockchain, then those Bitcoins should be yours until you spend them. The general rule of thumb is to wait for 6 confirmations (roughly 60 minutes) before considering the transaction as fully confirmed.