This just happened to me, so I will share the catch I made.  Please let this guide serve as an addendum to the other, more in-depth guides about spotting fake NF gear on the 'Bay.  Others have written nice detailed guides about spotting suspect sellers, etc.  Use their info.  Here is a way to make SURE what you get is legit.  Sure, you can look for stitch patterns and things like that, but unless you are REALLY good and looking for that stuff, and knowledgeable in terms of what stitching for outerwear should look like, etc, it might elude you.  My spot will not. 

LOOK AT THE ZIPPERS. 

North Face, and for that matter Mountain Hardwear, Cloudveil, Moonstone, Marmot, etc, usually have (exclusively in the case of NF) deals with the zipper manufacturer YKK, to install YKK zippers on all their products.  I got my NF Summit Series jacket home, it looked really really nice.  Sure, there were a couple loose threads here and there, but that happens.  As I was putting it on and trying it on, I unzipped the liner jacket and noticed the zippers on the liner jacket looked funny.  Sure enough, they weren't YKK.  This was interesting to me.  I grabbed the other 4-5 NF pieces of gear that I own, and on EVERY SINGLE ONE, there were nothing but YKK zippers.  This one, as I looked more and more, had not a single YKK zipper on it.  It is also worth noting that the pockets were weird on my jacket, but that was just a feeling for me, they 'felt' weird, till the zipper find confirmed it.

Look at the zippers guys and gals, it's a neon sign once you know what you are looking for.  Why do they use YKK?  I don't know, but they do.  I got lucky and spotted that, call me a huge outdoor gear nerd.  I hope this helps.