It never hurts to double-check your tuning. Play the lowest string and the D-string (the fourth string). It should be an octave higher than your lowes...
Have you ever wanted to play a song which is written in Drop D tuning, but you didn't have your tuner on hand? Well, this task is actually quite simple, and after using this method several times, you'll find yourself able to do this under a minute. In no time at all, you'll be able to switch between songs with a dropped tuning and a standard tuning quickly and correctly.
Step 1: Precautions
Make sure your guitar is in standard tuning. If your guitar isn't in tune, the end result will be less than great. If you're unsure on how to tune a guitar in standard tuning, check out this wonderful instructable, written by builder2.0. Once you're in standard tuning, move onto the next step.