Generally, if you are on the receiving side of the Bitcoins and the Bitcoins are there with 6+ confirmations you are unlikely to be scammed.
Most scams are technically unsophisticated and work on the reverse:
Send me your bitcoins first, and then you will:
- receive goods
- receive services
- receive interest
which of course will never happen.

Beware of lending and "investments", and only do Paypal with trusted counterparties. Generally if a deal looks to good, its probably scam. Don't be shy to ask why it's worth for the other guy. If he can't tell you a plausible story how he is making a profit on the deal or why its worth for him to do it, just walk away.