I just tested one today. You need a digital multimeter capable of reading the actual ohm value. But even if you do not have that, the problem with analog is, as you move the arm up and down slowly, the wiper can skip and cause erratic behavior. The older the sending unit is, the more likely this is to happen. Usually cleaning the mechanism will get the wiper working. I use electronic contact cleaner. Amazing the difference after cleaning.
I would think you are right though, you need a new 30 ohm sending unit.
