I've always checked my Tanita scale against the doctor's scale. Before I leave for an appointment I'll step on mine with my clothes and shoes and the whole trip. Then I'll go to the doctor's office and step on theirs. They always match exactly. If they didn't match I assume I would buy a new scale.
Based on old scales becoming less accurate, I would guess that the new one is right. I've been known to set dumbbells or kettlebells on them as a test. It's a pointless test because the scale always correctly reads whatever I set on it, but maybe if you're comparing two scales you could see if one or the other is off.