When it's on eBay, there's little to go by other than the price being asked for, and to a lesser extent, the seller's rating. Anything that seems too good to be true most likely is.
If contacting the seller, you can ask for pictures of the bottle and packaging. Most sellers just rip pics off other sites. I've seen fake Creeds that come in reproduced boxes. The often velvety logo on the box may be just a matte square, trim and accents on the box are different colors, etc. One bottle of Green Irish Tweed I saw being sold as an EDP, had an obvious stamp on the bottom of the bottle showing that it was a decant of an EDT.
Look for anything irregular about the bottle. Go to a store you trust first, familiarize yourself with the bottle. Is the logo etched or a sticker?, etc.
...and there's nothing stopping people from filling real bottles with fake juice too.
I'm sure other members here will have better tips.