It is not about how many moles you have. It is about whether one of them looks different from all the others. Paloma Cornejo, a dermatologist and head of the Más Que Derma clinic, calls this the ugly duckling rule - and it is the fastest test anyone can run at home after summer.

The logic is the reverse of what we expect

In most people, moles resemble one another: similar size, round or oval shape, roughly the same shade of brown. That is the normal for that particular body. When one mark stands out from that pattern - larger, a different shape, a different colour - that is the ugly duckling, and that is the signal to see a doctor.

It matters that this is not self-diagnosis. Standing out can mean seborrheic keratosis, an atypical naevus, a carcinoma or a melanoma - four entirely different diagnoses. The job of the person at home is not to guess which one it is, but to notice it.

ABCDE - and the letter that counts most

The second rule is well known: A for asymmetry - one half looks different from the other; B for borders that are irregular or blurred; C for colour, when one mark contains several shades; D for diameter; E for evolution.

She calls the last the most important. A mark that grows, loses its symmetry or changes colour needs an examination - regardless of how it looks at that moment. In other words: this is not a photograph, it is a film. What matters is what has changed since last year.

The places nobody looks at

She lists the areas that regularly get skipped: between the toes, the soles of the feet, the scalp, the genital area and the back. Of the back she says it is almost impossible to check alone - simply because a person cannot turn that far round. The solution is for somebody else to look, or to photograph it.

Another common misconception: melanoma does not have to appear on an existing mole. It can show up as a new mark that looks different from all the others from the very start.

Who should look more often

People who have already had skin cancer, a family history of melanoma, sunburn in childhood, a large number of moles or a fair phototype. For them a regular examination is not caution, it is routine.

The dermatoscope - the magnifier that shows details the naked eye does not have - is why a self-check is not a substitute for a doctor. But it is not useless either: it is what decides whether the appointment gets made at all. The end of summer is a good reminder, though Cornejo herself says any time is a good time. The question is when anyone here last booked an appointment for something that does not hurt.