In-store checklist when buying big/fast glass?

A Nikon 300 VR f/2.8 or 500 VR f/4 is what I am looking at buying.

Of course these both cost big $$$: so I want to check it out carefully at the store.

Any specific tips for ensuring I walk out of the store with a lens that that is a good sample?
Thnx, Srathglass.