Originally Posted by: ChristopherSchlegel

Pitch pipe

https://www.guitarcenter.com/Kratt/SN10-Guitar-Pitch-Pipe-1274228071684.gc

Tuning fork

https://www.guitarcenter.com/DAddario-Planet-Waves/Tuning-fork.gc

These great old school options require you to learn pitch matching. They don't explicitly show you that you are in tune, you have to judge for yourself by the sound of the tuner & the sound of your guitar string. But they provide a reliable, external reference source for tuning.

Hope that helps!

I still have my A440 somewhere. Great little device. lol