Here at Proaudioland.com we have a nice selection of tuners but if you haven't had a chance to pick one up or if you find yourself in a situation without one, take this tip from our friends at Fender:
You need to tune up, but you don’t have a tuner. Or you do have a tuner, but it isn’t working for some reason. There isn’t a piano nearby or another guitar that’s already tuned or a singer with perfect pitch. No worries. Just pick up the phone.
The dial tone on most U.S. landline telephone systems is an “F.” Just tune to that.