A hydrometer measures the weight of the water (specific gravity). Salt water is heavier than fresh water. It's a plastic thing you'll find near the salt at LfS
. If you go reef at some point, you'll want to get a Refractometer, which does basically the same thing but is more precise. The hydrometer will get you by to start, and I think its usually $10 or less. You'll want specific gravity to be between 1.021-1.023 for fish only tank and 1.023-1.025 for corals. Ocean water is 1.026, so you can decide for yourself which number you wanna keep it at. If you're new to salt, better to keep it a little lower at 1.022-1.024 cuz when you have evaporation, your specific gravity (salty-ness) will increase. If you're a little on the lower side, then you'll have some wiggle room before your fish are adversely affected.