Hi Jon,
First of all, check the price differences between the brand new after discount and the used price you are getting. If not much different, i will suggest to save up for a new one.
Otherwise,
1) Check if the watch has the same serial (usually can see from the caseback) number as stated on the warranty card. For example, Omega, each modern omega will have a serial number at the 7 o'clock lug. (Some fake do have too) but at least, that's the thing that you must check.
2) Send the watch back to service center for authentication. Rolex and Omega service center willing to help but may charge you.
3) Buy the seller, if you are not familiar with the seller, get references otherwise avoid dealing with him/her.
4) A lot of used watch shop around town, I would suggest you check out the shop first (at least you can touch and try and get warranty from the shop).
Hope this help, i'm sure others will have better point of view

Good luck!
scott