But you can at least test how different pickups compare to each other on the same guitar. The general characteristics are going to transfer between guitars, so if you've already got a similar pickup in another guitar, then you will know what one of the other tested pickups should do to the sound. If you choose one pickup (of each general type) as a standard, then you can compare the others as to the relative output, plus the different amounts of bass middle and treble they have compared to that.

Of course you are going to need some test equipment if you are doing it objectively, with at least an oscilloscope to measure signal levels and a frequency analyser (most DAWs have them as standard).