Before the age of electronic tuners, I used the guitar. The lowest note on a standard tuned 'harp is F, the second lowest note on the guitar. The highest note on a standard tuned 'harp is C, found at the 20th fret of the guitar, often the highest note on the guitar.

Now that electronic tuners are available, I have lately been using a red Snark. If you have an extra tuning wrench, you can put it on a post and clip the tuner on the metal part of the wrench. If you just have one wrench, clip the tuner onto the metal part of the wrench you are using. I always give it a last minute fine tuning before each show or jam.