In some older cars an air flow sensor can be tested with a voltmeter. This procedure involves measuring signal voltage at different engine speed and comparing it to specificaed range. In most newer cars, however, an air flow sensor produces a digital signal and the only way to check it properly is to test it with a scan tool. If you have a good scan tool that shows live data, you can check mass air flow readings at different rpms and compare them to the specifications for your car or to the measurements from a known good MAF sensor at same rpms. Typically, a contaminated air flow sensor will show lower air flow than a known good sensor. 

Sometimes poor electrical connection at the air flow sensor connector could also cause the air flow readings to be out of range. For this reason, the air flow sensor connector terminals as well as the wiring needs to be carefully inspected.