give me an E or an A, and i'll take care of the rest. i use relative harmonics but you need a starting point if you don't have perfect pitch. one guy on here constantly brags about tuning by ear but i'll bet you anything he's always out of tune. one of my friends is like that "i have a musical ear" and "i can tune by ear".
his hamer explorer and jackson V guitars are always horribly set up and strung improperly and drastically out of tune, and he actually has, no bull, told me that it's a tuning he made up. to retain credibility i guess. but i've never been that impressed with people who use altered tunings that happen to be a semi-tone under concert pitch with the A totally slack, and the b tuned to E#.
if you don't have a tuner, you're either incredibly musically gifted or just pompous and ignorant (and out of tune).