My KidsKnit group has LOTS AND LOTS of "unmarked" donated yarn. When we get to a place where we need yard measurements for projects, we will do what Ingrid suggested, using a scale from the Science Lab. Somewhere .... but of course I can't find it there is an article about the "granor"??? scale that is used to weigh yarn. Right now, we are learning to cast on, knit ....and not use our sticks for anything but knitting We have three rotating groups ... one learning to knit, one making needles, one winding center pull balls. That way I only have to answer and help two at a time while we are learning!
I did find this :
An average worsted-weight yarn will have approximately 50 yards per ounce, Sports yarn 120 yards per ounce and baby yarn 150 yards per ounce.
Dark colors take more dye and will weigh heavier, so expect the darker colors to have a little less yardage per ounce.