I have to tune thirty guitars at the beginning of each lesson because the kids think it's funny to detune them when they've finished. I discovered that, with practice I can correctly judge the A (I sometimes use a pre-tuned guitar of my own. Then I match up the D string by singing 'Here comes the bride' to the A string which starts ADD. When it sound right I do the same on the D string to give DGG then you need to play GB the first two notes of 'Kumbaya'
then back to 'Here comes the Bride' for the E string - play BEE. The top E is now two octaves away from the low E so try singng 'Somewhere over the Rainbow' the first two notes of which will be E and E'. That lot gets me through thirty guitars in around ten minutes. (In concert I use a headstock tuner like normal people!)
cheers
Paul