I'm assuming you know the simple way, fret-lowE-5th for A, fret-A-5th for D etc .... well, just drop back a fret when you do it .... when I'm doing it that way I'll start de-tuning the A string first, down to the high E string, then i'll play low/high E strings simultaneously and de-tune the low E until they sound right.
You can then check the tuning of lowE more closely by fretting back at the 5th fret again, you should get a note matching the G# (can't do the flat sign on here :D) of what's normally A.
(sorry if the wording doesn't make sense)