One telltale sign is that the bark has loosened its hold, or has already been knocked off with handling.
Also, the log ends have darkened, dried out, and started to "check" (crack), not to be confused with the deeper split marks from an ax.
A well-seasoned fire log will be lighter in weight than a partially-seasoned or "green" piece of the same size and species.
When it really is well seasoned, expect to pay more. Cutting trees down, transporting handing, and working up wood is a risky, labor-intensive pursuit; any do-it-yourself wood-burner will testify to that. The more times a supplier has to handle it and the longer he ties up space storing it, the more he'll charge. And rightly so.