Do you want them on your team next year is a good start
But seriously, best thing to do is probably looks to see who you would like a to keep for a few years and where you would be drafting them. ie 10th rounder, then 5th, then 3rd, etc. If you thinking drafting them at those spots would be good value relative to where you would have to draft them normally, you should highly consider them as your keepers. I like am keeping Doug Martin because I can keep him in the 4th, 2nd, 1st, etc and would have to draft him in top 5 of 1st to get him normally, so he would be a good keeper.