Bill, this is a topic I have some familiarity with. I would say that Gene and Joe are both pretty much right, but Gene's advice is much more pertinent.(posted 8527 days ago)With a powdered mix like Xtol (no liquid additions), checking the specific gravity is really more of a check for mixing errors than anything. These errors are basically either 1) Kodak put the wrong amount of powder in the mix, 2) you used the wrong amount of water, or 3) you didn't get all of the powder to dissolve.
To put the specific gravity test in perspective, if a practical joker were to sneak into your darkroom, dump out the Xtol and replace it with an equivalent weight of sugar, for example, it would almost certainly still pass the specific gravity test. However, Gene's "clip test" would still catch the problem.
If you have a scale, like Joe suggested, you would probably be as well off to just weigh each package of Xtol and make sure they're within about 3 or 4%; then be careful that you don't make a blatant error mixing.