I constantly test frequency generating equipment, even the good stuff goes fluky!
Just tested my 2 mini-FG's again, 1 of them had signals, voltages and current all over the place on the P3 setting which is the square wave, positive offset at 9V.
Wasn't even close. Then we wonder why we have little success or get ridiculed.
Anyway, just a decent Voltmeter from the Shack or other electronics parts store. These are easy to use and gives enough information without going into oscilliscopes and spectrum equipment.
Measure the voltage coming off the 2 ends. Learn to measure the current, let the dude at Radio Shack show you how. If you get a decent meter, it will show a Hz setting too. This is pretty accurate. Set your FG to a particular frequency and put the meter on the Hz setting to make sure its delivering the goods. Some meters even show amplitude and things of that nature.
People with those cheap 'zappers' need to check them constantly. I try to post in the zapper section but get to many e-spankings for even mentioning technical stuff.
Try to buy or make 2 of everything if possilbe, this gives a better baseline.