There are several ways to evaluate pickups for tone.

• The bottom line is of course how it sounds with your guitar, sound equipment, and listening environment. This takes a lot of trial-and-error and in the end, the results are very subjective. So some science could be helpful. Maybe.
  
  
• Online audio samples from different pickups are useful. The exact pickup locations and string heights are unknown. It won't be your guitar or strings and it may not be your playing style, but still you can get some idea of the pickup tone.
  
  
• Manufacturers use terms like warm, sweet, big, strong attack, deep, fat, thin, harmonic bite, hard-hitting, harmonic quality. I guess they have to say something, but these qualities are very subjective and hard to quantify.
  
  
• Manufacturers often provide low/mid/high tone levels, total output level, and self-resonant frequency. I think these are useful for comparison of pickups from a single manufacturer. I am wary of comparisons across different manufacturers, though, because the test setups that gave these numbers are almost never carefully defined. In other words, the measurements can be tailored to give almost any results you want to see.