I've held forth on this subject in the past so I'll just include links to what I've already posted. As HuwG and others have pointed out, you may be forced to tune to something other than A-440 standard pitch. These tips will help you tune quickly and accurately in any situation. Even if you are at concert pitch, your guitar may not sound quite right when tuned to mathematical precision by an electronic tuner.

Discussion of avoiding harmonics and learning to listen for the “beat” created by overtones.

Discussion of tuning as a practiced skill and the mechanics of doing it quickly and accurately

The post at the first link actually contains a link to the second link but I include them both for the sake of completeness. The method I discuss must be learned by practice but once you have it you'll be glad you took the trouble to learn it.

      - Mark