One thing you could try would be to check every spoke with a tensiometer to see how close to equal they all are. alternatively you can try the musical plucking system to see if they all sound about the same.
Assuming you want to fix it yourself and don't have any special tools, try this:
1. Loosen every spoke until you have exactly 1 spoke thread showing. That'll take all of the tension off of the wheel but, assuming the spokes are all the same length, they'll be equal.
2. Now spin the wheel. If you're seeing a lot of side-to-side wobbling, more than 1 per revolution, that means that your rim is bent. If that's the case, your best bet is probably to replace the rim with a new straight one.
3. If the rim looks pretty straight, tighten each spoke 1/2 turn at a time and go around the wheel multiple times to slooowly bring the wheel back up to tension. Any time that you save by hurrying at this stage you'll pay back (with interest) during final trueing. When you think that the tension feels about right, go around the wheel squeezing parallel pairs of spokes together to set the elbows and nipples. Do a final trueing on the bike by tightening and loosening opposing pairs of spokes an equal amount and you'll be good-to-go.