No, you cannot really test the rate controller without product actually flowing through it. However, you can test your valves and your speed input by just doing a couple of simple tests. If you open your control valve by using your manual control switch you can test to see if the control valve works and you can also do this similar test with any of the on/off electric valves you have in the system. Then by driving the tractor with the applicator attached around your yard or nearby field you can make sure your controller is displaying speed. If you do these things you can be assured that the control valve, speed, and on/off valves should work before you attach a tank to your applicator.