Hi. I spent twenty years developing an almost infallible ear....but now I'm so lazy it's revolting! Use an electronic tuner, and do what Clinton has suggested. You'll be surprised to find that it's probably out a hair at several frets. The guitar is an imperfect instrument, but if it's more than a couple of cents out you may wanna get your nut or saddle re-cut. If it's out by more than that (like on most older Martins) you may want to do what I've done on at least 5 good guitars and that is have the saddle moved (or split). I'm a fanatic about being in tune.

Rick