If you're able to watch their testing methods, etc, I personally don't see any problem with letting the LFS do your tests. I used to use my LFS too, since I was already going there for other things so it made the job easier... and since I knew the test kits they were using and could watch them do the tests, I trusted the results. Of course, I also do my own testing, so I know what to look for when they do it.
At any rate, you should probably try to hunt down the source of the hair algae, which is likely phosphates being present in the water. It's an extra element that's likely getting into your water somehow, either at the water source, or through your salt. Are you using RO, RO/DI, or strait tap water? If you're getting RO water from your local Walmart (or something to that effect) they aren't likely doing De-ionization, a process which usually gets rid of minute amounts of phosphates that can cause the hair algae to spread. Tap water is just a bad idea unless you're really experienced and are testing for everything under the sun (the tests you need to do on tap water to properly use it aren't things sold at your LFS, but most along the lines of hardware). RO/DI water is your best bet to prevent hair algae since it will get rid of 99% of contaminants that cause problems in SW tanks.
You should test (or have your LFS test) your water source prior to mixing salt, as well as your mixed salt water, prior to it's introduction into your tank. I would bet that one of these tests will yield your source.
