Refractometers and conductivity meters are the more accurate ways available to hobbyists to test salt concentration. I have both. I tend to use the conductivity meter more, but they're also more expensive.
i had a hydrometer at first as well, but after I purchased a refractometer I found out how easy it is for the hydrometer to give you an inaccurate reading. A refratometer will give you an exact reading and it doesnt rely on moving parts that little bubbles can stick to.
I used a hydrometer when I only had my sw fish tank, and thought it was working fine. Last week I got a refractometer and found that my salt was at 1.020 when my hydrometer indicated it was 1.025. It's best to pay more for something that will work accurately. There are many important things in this hobby, and salt content is one of the top.