You should test your equipment before you use it. It is also important to check your measuring devices it make sure they are in good working order.

You can do a google search and find plenty of ways to test hydrometer.
It can easily be done with a room you know is dry or you can place it in a zip lock bag that has a damp cloth it should be almost 100%. If you put it in a sealed container with a bottle cap full of really salty water it should read 75%.