the first thing that I like to do is spin the bass on the end pin as fast as possible. Less wobble indicates the bass is well-balanced symmetrically and worthy of further consideration.
If that goes OK, I next tilt the bass over a bit and throw my leg across it and "get in the saddle" if you will. I find the wider lower bouts to be more difficult to mount.
If the bass passes that test, I will go forward with the full C bout mount. For this I make sure I am wearing rubber-soled shoes, as the newer, shiny basses tend to be slick and a leather soled boot or shoe slips easily. If the bass balances well, a couple of pogos around the shop usually tell me all I need to know about the bass.
But it is really about the sound, so if the bass is still cutting the mustard, I'll drop to one knee and throw it up on the other and crank out a bar or two of "Rock This Town." I have found the Gamba style to be more conducive to this practice, as the pointy corners of the Viol shape can cause a pretty nasty charlie horse.
