I've read a few of these guides as to spotting fake perfume: Check return policy, see if it's greasy/evaporates too fast (too much oil/alcohol in fakes), if out of 1000 pos feedback there is 1 neg saying they sell fake - this is probably the accurate one.
However, the quickest, simplest way for me is to simply check the serial number on the bottom of the box (and bottle).
On the box the serial number should be indented (NOT printed) on the bottom of the box (usually under the designers name). The number should be the same as that on the bottom of the bottle.
Nb. It is not necessarily always indented on the bottle. I have a bottle of Armani Diamonds bought from a reputable dealer, the bottle has an Armani hologram sticker with the serial number on it (on the bottle).
If you buy a fake from a seller on eBay - Report them and Leave Negative Feedback!! Warn other buyers!!! Plus, recently eBay has introduced a policy where sellers can't leave neg feedback for buyers - to encourage true reviews and stop retaliation feedback. So now there is no reason not to warn fellow buyers!