I've been in a stockinette sock slump again lately. This time I was carved into that rut so deeply that I made four matching socks. They're all knit from one 100-gram ball of sock yarn (Opal Cotton).
How did I know I had enough yarn left to knit a second pair of socks? I used my kitchen scale. I've got a good digital one that will measure in grams or ounces. When I finished the first pair, I popped the rest of the ball on the scale; since it still weighed 54 grams, I knew I had enough for a second pair. By the way, I would have run out of yarn if I hadn't made them fraternal quadruplets (they're not quite identical, since the subtle stripe pattern falls differently on each sock).
A postal scale will also do nicely, and if you don't own a scale, all you have to do is stop by a post office with a self-serve scale in the lobby.