Refractometer is the most accurate. But depending on what you are keeping hydrometer works. The old glass ones are pretty easy to use if you can find one. The catch is you need a taller container to float them in because they bob up and down in the current.
I started out using a hydrometer for years. It is the cheaper option.
refractometer for sure. Pico tanks are a daily care thing, so always top up with fresh water to keep a solid salinity in the tank and make sure your new water is the same temp and salinity as the tank water when doing a daily water change.