Unfortunately, there is no quick and easy way to know. I made about post as a suggestion to make this much faster an easier. Basically, you have to know how the overall formula works, then do a bunch of math to compare each new item. I can give you a few general tips though:
* With the exception of toughness, almost every statistic is subject to diminishing returns, which means the more of it you have, the less of a benefit you get from it. This means that getting items with stats that you don't already have or are already low tends to be more helpful.
* The exception to this rule are the stats that you have additional boosts from in your passive tree. For instance, while 50 dodge score might give you 5% dodge chance and 100 all resistances might normally give you 5% resist to all elements, if you have +18% resist all nodes, then you really have 118 all res and thus all res becomes better. In general, if you stay with the recommended type of armor for your class (heavy, bruiser, rogue, sorcerer), you should get the most benefits from your passives.
* You also have to keep in mind that for attributes, you get something like 2.5% or .25% bonus damage from your main attribute and 1 or .1 from your secondary.etc. (it stats the amount on the character sheet). This means that for attributes, even though you are getting diminishing returns, you will likely still benefit more from points in your main stat than from others (though the toughness defensive boost is higher than the offensive boost of any other attribute).
I hope this helps!