If you had a piano, you could just match the keys there by ear, and standard guitar tuning only needs six: E, A, D, G, B, E, running from the lowest bass to the highest treble.
If I tune the low E to an external source, then tune the other strings from that? I'm usually a hair flat or sharp on subsequent strings, more noticeably the trebles. Not that much, but off enough so using a tuner to check immediately shows it.
Better than I used to be, because I practice, but the wetware and input speakers are what they are ...
The future arrived, did you notice? Now you can get a clip-on tuner that works via vibration and is chromatic, i.e., you can tune any fretted instrument using it, with a range of starting tones. And cheap, too.
Mine comes with backlights. Red, it's not there; green, it's tuned. (Though you should note that a plucked string, because of how it works, will often read sharp or flat for a beat, then settle in tune.)
The standard A440 can be dialed up or down to match sharp or flats. In a lot of old recordings, the players didn't use A440, though they were in relative pitch, so if you want to play along, you have to adjust your guitar, and the tuner can be set to do that.
When they talk about these things, the term "cent" is often used. A cent is one-hundredth of a semi-tone. Apparently the average human ear can detect a difference of five cents or so, and if two guitars are ten cents apart, somebody is going to be noticeably out-of-tune even to guys with ears like mine.
Most of the common electronic tuners are accurate to ± 0.5 cents, well below what is needed for most ears. There are folks who are more accurate than the machines, just as there are drummers better than a click-track. Gifted folks.
All of which is to say that my guitar has sounded a little off the last few days, and what I noticed when I tuned it yesterday was that the tuner control had been bumped and it was set at A442. When I reset it to where it was supposed to be, it sounded just fine. Very strange that I would notice two cents difference; I'm not sure what it means.
For more on this subject, try this link.