As far as players go, you will just have to try and be the judge on whether it's fair or not. Sometimes, IRL, teams make horrible trades (Roy Williams was traded to the Cowboys for a 1st and 3rd rounder, Randy Moss was traded to NE for a 4th).
But, if you are looking at strictly draft picks, this might help:
http://www.calculatorsoup.com/calcul...pick-value.php
In there it has a calculator to see if it's a fair deal and a chart to see what each pick is worth. As a rough guideline, next year's picks should be valued about one round lower (that's why you see a lot of teams give up next year's 1st round pick to move up in the 2nd round of the current draft).
Teams don't treat the chart as law, but it's a good place to start.