Other than using songs that happen to contain a fifth interval, as some folks have already mentioned, you can sing a scale to find the notes of the strings.

You know, the old "Do re mi fa so la ti do" thing?  If you tune your A string to match the tuning fork, and sing "Do" to that note.. then sing your way up the scale, "Do re mi fa so.." and "so" will be the note for your E string.

Then you can sing down the scale from the A to get your D.. "Do ti la so fa.. "  and "fa" will be your D. 

Once you have those three, you can play the open "doublestop" of the A and E string to fine tune the E, if needed.  Then do the A and D.  Basically playing the two notes together and fine-tuning the one that isn't "A" a little (if necessary) so you don't hear a "beat" or "wobble" between the notes.

Then when you've got the D string tuned and fine tuned, you can sing down from the open D, "Do ti la so fa.." to get your G.

I don't know as there's any big advantage over using the first two notes of a song that conveniently has the right notes.. Other than finally having something that "Do re mi fa so la ti do" is useful for, if you had to learn it back in gradeschool. LOL