See the box got dust or not.... :bsmilie:
Seriously, the salesman will normally take out the lens from the box, out from the plastic bag, then hand it to you.
You are supposed to check the outside got any scratches or not, then check the ink marking, then open the aperture at the widest, (ie f2.8, f4 etc), then point the lens to a bright light source, (eg sunlight outside the shop, light tube on shop ceiling), then look through the lens from the camera mount, and see got any fungus inside, then see got big dust inside (small dust is common and no need to worry), also see got hair inside or not. Then turn the lens around and see from the front lens and check fungus/dust/hair/etc.
Remember, the salesman are there to sell u a lens, they will try whatever it takes to sell it to u, believe them or not is up to u, dont buy under pressure.