Fiddlerman said
I alway use my ears for fifths but what I used to do with my students is suggest that they first tune the strings in their preferred way, for example a tuner, then play the fifths together to fine tune.

@Fiddlerman This is exactly how I tune.  Tuners, although mathematically right on the money, are not the same as my ear.  And listening to the fifths trains my ear anyway.

My viola friend wants me to tune entirely by ear.  I will probably do that eventually, but I am not that good yet.