This is a great question, because how long do you give it? And is it possible that both teams can win the trade? For example, the Tommy Pham deal. Is it at all possible that both the Rays and the Cardinals benefited from that deal? Or, is the deficit of production in the Cardinals' outfield such that you cannot grade that trade on that participants alone and have to factor the hole not filled behind Pham's departure. It gets tricky.
Consider the Mercado deal. Cleveland got a starting CF. Do you remember who the Cardinals got? Well, in that deal it's Jhon Torres, who could be a top 100 prospect in the coming two years -- top 100 in all of the minors. So, who won that deal? Cleveland got major-league production. Cardinals got ... the less tangible hope of a higher ceiling.
One surefire rule of thumb that I have with trades is if the trade catapults the team to a pennant or a World Series championship it was likely worth it.
So, in 2011, the Colby Rasmus deal was worth it, regardless of what followed.
You bring up a good corollary to that view: If the team's need was so great that the trade had to be made and a loss was likely. I think the Marcell Ozuna deal, in the end, is a win-win for the two sides. The Marlins will get more from that trade than the Cardinals, most likely, but the Cardinals got the immediate bat they needed, and they didn't walk away empty-handed given that they also got the draft pick. Goldschmidt works in this same way. The Cardinals traded from an area of depth to get a player who changed their lineup and became a face of the franchise.
That's win. So, maybe there's where you start.
Did the team trade something it will miss or only had a few of on the roster for a player needed and was the cost slightly more than the return. If so, the trade worked.
One trade that will take some time to sort out is the Arozarena one, for sure. The immediate return certainly favors the Rays and suggest they cannot lose the trade. The Cardinals, however, can yet call it a draw...