Have you been to a GC? Or just a guitar shop.. That notorious question comes up.. "Hey there, you think i can have a patch cord and try out that bass?" You then plug in...you think to yourself...."hmm. lets tune... now, what song to play." What kinds of things do you play?
I always start with a huge slide up and down the E string followed by a ringing open E, just for kicks
Then I will do octave runs and the major scale up and down the neck to get a feel for the bass and its general tone etc.
Then I might poorly slap the E string and try some tapping. I do this in the hope the bass magically improves my technique, since I never practice them enough
Songs-wise I change up what I play depending on what I am learning at the time but these tunes seem to be staples for me when trying out a bass:
Intro to For Whom The Bell Tolls (Metallica)
Verse riff to Hail Hail (Pearl Jam)
The bridge to The Chain (Fleetwood Mac)
Intro riff to Blood Pus and Gastric Juice (Pungent Stench)
Bass part to Orion (Metallica)
Chorus to Under the Bridge (Red Hot Chilli Peppers)
Also if someone tries to jam along with what I am playing I immediately stop playing and move on to something else. Just a habit I have - I don't really feel comfortable jamming in a shop - We're there to test the stuff out for a purchase, not use their stuff for an impromptu jam session or worse a competitive wank session.