[gpm]

> How would one otherwise be "sure it has not been infected"? Any feasible ways?

If you are willing to assume they aren't infecting every computer. Walk into a random brick and mortar store and buy it there.

If you're paranoid to the point where you don't trust the people at a random brick and mortar store, point at a display model (or if they have non-display models visible one of those) and insist on that one in particular, without it leaving your sight at any point in time.