The warning signs have a neat acronym: ABCDE.
A = asymmetry. Normal moles are pretty symmetrical, the dodgy ones are a weird shape.
B = borders. Normal moles have a pretty well-defined border. Dodgy ones like mine have hazy borders. If you zoom in on mine, it looks smudgy round the edges.
C = colour. Normal moles are mostly a pretty even colour. Dodgy ones have varied colours. Mine had a much darker patch.
D = diameter. Bigger moles are more worrying. That's the only warning sign I didn't have.
E = evolving. Rapid changes are a cause for concern and readers, this dodgy mole looked ABSOLUTELY NOTHING LIKE THAT a year ago. It was a neat little mid brown circle.
So anyway, see a doctor if your mole is fucked up in any of the ABCDE ways because apparently it takes forever for them to take the damn thing off.