You can even use the keys of a piano or any instrument that produce the same tones used by your instrument.
Several things can cause improper tone to be produced by your instrument.
When not in use, tension on the neck by the strings can cause the neck to bow.
Most manufacturers provide a few tools to for neck adjustments, and for peg adjustments.
Yes the hardware can, on some instrument become loose, producing lower tones and misadjustment, even while strumming the strings. (Also see, www.electric-guitar-info.com)
Also strings can stretch minutely and change the tones produced.
Producing the correct tones is very important, especially when playing with other instruments.
If you are not sure, have someone familiar with the instrument do it for you.
We will for simplicity deal with standard tone on each string, where from the bottom string high E to B,G,D,A,and low E on the sixth string.
However musician use many alternative tones on certain strings, producing different effects, which will be discussed in a further lesson.