determining length of skein

Last week I was seduced by a yarn color at a craft fair. I asked the woman who was selling it how many yards, and she said "I don't know but it weighs 4 ounces." So I bought the skein of yarn and brought it home. Now. How can I determine how many yards are in it, without actually measuring the entire length? The skein was about 22-24 inches long when I laid it out on my countertop, so I roughly guesstimated that it was about 72-74 yds. It did have a lot of tight little bumps or overspins from her spinning, so it is probably a bit longer, but I hate to start knitting something and run out! I'm guessing it would be comparable to a worsted weight, or one class lighter. Any help that you can give me would be greatly appreciated!!! Thank you.