Unless you got an anechoic chamber you aren't doing this on your own. Manufacturers are required to publish the angles and gain of their antennas, they often give vertical horizontal and azimuth patterns showing what the footprint is. When they measure an antenna beam width they use the 3db down points. So from the 3db down point of the entire beam width that is the angle they measure (and publish). Go to the antenna manufacturers website and pull up data sheet on products. As a general rule above 8.5dbi of gain they start becoming patch or sector antennas and not omni's.