Good thinking Elegran. Another way is to get crafty Nanapug, they are. Read the ball wrapper and see if you can find the tension it knits to, or pick up and read a pattern that uses the same yarn to find the information you need. Failing that, most Double Knit yarns need size 8s and size 10s, most 3 ply yarns need 10s and 11s or 12s, so the number of stitches required to produce a square of 2" or 4" will help you to work it out. They don't tell you the ply so you have to buy the yarn specified, they hope.
Of course if it does not knit up to the size square specified then you just change the needle size, one size up if too small, or one size down if too large, as needed to get the tension specified. It takes time but it is worth it to have a professional looking garment at the end of all your hours of work.
It also helps to buy an extra ball of yarn because if you need to use a smaller needle it may take more yarn to knit it up. Just remember to read every word of the pattern and make a note of any changes needed to achieve the result you want before you start to knit.
Sorry if this is more information than you needed but I can only guess at how much you already know. 
