Well any fishkeeper should have a proper test kit. You should get yourself a good liquid test kit, like the API Freshwater Master Test Kit. It will allow you to test the most important tank parameters: pH, ammonia, nitrite and nitrate. For the puffer, since you might eventually be converting him to full saltwater, you'd probably need a saltwater test kit. I'm not sure exactly which tests are usable in both conditions or at which point you need to switch to a saltwater kit when you're going brackish. The liquid test kits are much more reliable than the paper test strips, and they offer you many more tests because you only use a few drops out of the bottle at a time.
The hydrometer is a physical device that measures the salinity of your water, so you can keep using that forever (well, until it breaks I suppose). You'll want to make sure you get a hydrometer that goes all the way down to 1.000 specific gravity, since the fish is starting in freshwater and will need to be very slowly changed to at least mid-range brackish water.
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