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.
Step 2: Tuning
Place your index finger on the seventh fret of the low E-string. What you need to do now is tune your guitar so when you play the seventh fret on the E-string (the sixth, or lowest sounding string), it will play the same note as the open A-string (the fifth, or the second-lowest sounding string).
Step 3: Conclusion
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 lowest string. In other words, the open fourth string should sound identical to the lowest string on the twelfth fret. If it sounds right, congratulations, you're done.