Re plating of Penny Blacks.. these days I can manage to spot plate 1a and 1b but bad eyes make others pretty near impossible. You need a decent book or reference to get to know the plates. For "easy" ones like 1a and 1b the "ray flaws" (top left star) are normally all you need... but an expert can spot subtle differences that matter greatly.

This image is useful only as an basic illustration of what to look for.
Always get a
second opinion anyway from an expert. It might make a vast difference to the selling/buying prices.