When you play a note, the tuner picks it up and usually has little lights, and a needle to tell you if you need to tune it down or up. If it is plug in it takes sound in from a cable. If it is not it...
I'm not sure what a guitalele is, but tuners typically now have a variety of settings and there are settings for chromatic tuning, so if you know the notes the strings are supposed to play, you can...