auctioneer addon, it allows you to track historically what your item went for.

but mostly its down to experience, after a few months of playing, you get a feel of how much a item is worth, and once you know which stats is useful to which class, you will know better wether a item is worth selling

eg
plate items wiht +spirit, only paladins can use it, but +spirit is totally useless to paladins...so..the item obviously wotn go for much..just vendor or DE

alot of useless blue/green items is also commonly bought to be DEe'd, so they are normally the cost of the equivalent DE mat