Back in the early 1980s I used to use an A440 tuning fork as a reference to tune my A string and then tune the other string by harmonics, where the harmonic at the 5th fret of the E string is brought to the same pitch as the harmonic at the 7th fret of the A string, and then tune the D and G strings similarly.
Thankfully this is no longer necessary as there are a lot of accurate tuners available. I own several but my favourite is the TC Electronic Polytune Clip. I have two of these, a white one and a black one.
Although the Polytune clip has a mode for tuning a guitar polyphonically, I keep mine in the bass setting which has strobe and chromatic modes for accurate tuning.