The plain band is the honest test of a metal. There is no setting to distract, no plate to flatter the surface, no hollow construction to reduce the weight. What you see is the alloy, and a band that costs $10 is either hollow, not sterling, or both.
Ours is solid, 3 mm wide and 1.5 mm thick, which is a conventional width for a comfort-fit band and enough metal to survive being worn daily. The 925 stamp is inside the band where you can read it with a loupe or a phone camera.
Sizing: these are whole sizes plus a half. A 3 mm band fits true to a jeweller's sizing set. If you are between sizes, take the larger — fingers swell in summer and a band that is too tight is uncomfortable in a way a slightly loose one is not.