+1 Big Ape isn't the only one who gauges tension by sound.
Full disclosure: I'm a no-talent hack, so I tension by feel, but there are tools to measure the tension, and many modern rims will have a max recommended tension spec, although finding those specs for old wheels will be difficult and in most cases, impossible. I've spoken to and read a lot of old-timey wrenches who say that, BITD, they used to tension the spokes til the point where the wheel just began to taco, and then back off a quarter turn, but that would make me nervous. Personally, I just tension them til they all "feel" right to me, which is completely not scientific, and it's impossible to describe accurately on a forum post. Just remember that, on a non-dished wheel, you want the tension to be as close to uniform as possible on the spokes, and you'll want the spokes to be as tense as you can get them without undue stress to the rim or the spokes at the bends....