The only thing that putting bio balls from a freshwater filter into a saltwater tank will do is kill the bacteria on the bio balls. Saltwater reef tanks need different sand, rocks, light, water flow, and filter media and equipment than most freshwater tanks. I don't actually recommend using bio balls in a reef tank. If you want a fish only saltwater tank bio balls may be useful.
meter mentioned above is a great idea. They're cheap and really do tell you how pure the water is whether you have your own RO
filter or buy water from a store.
Again, if you're just starting out, reading a book on saltwater tanks is a great way to get some basic information about what it takes to care for your new tank.