Salutations!

I understand that I can use YouTube for tuning, but what if I don't have internet? I sometimes lose the internet suddenly, in the middle of my digital lessons. I also know that I can tune without a digital tuner. I press the fifth fret lower e string and hit the a string and listen to it until it's sounding the same as the lower E string I'm fretting. The problem is, what if my lower E string is out of tune?

Thank you.

- Mikael