Hach Phosphate PO-19 Color Wheel Test Kit
This is a very high quality, accurate and easy to use test kit. It's a rather large test kit, the size of a small tool box and its easy to get scared by all the parts. However, it really is a simple test. Fill a tube, add some powder, wait a few minutes, and compair color to a color wheel. Not very hard to complete. I must admit though that finding the right color on the color wheel can be hard as the gradient on the wheel is very fine.
These kits are not cheap, but once you buy them, you only need to buy refill kits which are very cheap.