There are two options. Aquarium Water Testing / Aquamedic is what I use. They test for:
Calcium
Alkalinity
Magnesium
Ammonia
Nitrite
Nitrate
Phosphate
Iodine
Potassium
Strontium
Molybdenum
Copper
Silica
Plus Iron and Boron as an additional test.
Triton is the other one and I think it involves sending water to Germany, unless I misunderstood.. As I get my AWT results back in 2-3 days and I trust them, that's what I use. I send off a sample every 2-3 weeks. I sent them a sample of fresh mixed Reef Crystals a long time ago and the results came back perfect. I think that the results I get match pretty closely with what's going on in my system, and when I make changes based on the water test results, I get positive results with the system. I think all test kits are marginal at best, and I think whatever AWT is doing is better.
Is it perfect? Probably not. But I think it's really good and the price is reasonable. I'd get a box of 12 for $350 (that includes shipping the samples to them) and send one every two weeks at first, assuming you'll need to make adjustments, then once a month.