Just look at your skill level -
Red items always give you one skill up per combine
Yellow items average around 80% skill up rate (figure 4 skill ups on 5 combines)
Green items run 5-10%
Grey items give no skill ups
So assuming that you can go with nothing but red combines you'll have a good idea how far you are from your next level.
Now just because red items always give a skill up doesn't mean that they are what you should always make - if a yellow item only takes 4 peices of leather and a red item takes 8 then I use the yellow item till it goes green since even though I'm not getting a skill up every combine, I'll get more total skill ups for my leather.
In other words, use your common sense.
As a side note, if you have an enchanter alt or a friend or guildy who's an enchanter, see about getting your practice peices disenchanted so you can sell the dust and essence since most of the stuff you're going to make to skill up off of wont sell on the auction house.