People who have more than 11 moles on their right arm could have a higher risk of skin cancer, research suggests.
Experts say they have found a new way for GPs to quickly assess whether somebody may be at risk of developing melanoma.
Counting moles in a "proxy" body area such as the arm is a good marker for spotting potential problems, according to researchers from King's College London.
Around 20% to 40% of melanoma is thought to arise from pre-existing moles and having more than 100 moles on the body is a "strong predictor".
The study, funded by the Wellcome Trust, examined data from 3,594 female twins.
Specially trained nurses from St Thomas' Hospital in London performed a mole count on 17 areas of each person's body. Skin type, hair and eye colour and freckles were also recorded for the research.
The results were checked against a further study involving men and women.
Scientists found that the count of moles on the right arm was most predictive of how many moles were on the entire body.
Those people with more than seven moles on their right arm had nine times the risk of having more than 50 moles on the whole body.
Those with more than 11 on their right arm were more likely to have over 100 moles on their body.
The experts found that the area above the right elbow was particularly predictive of the total body count of moles.
The legs were also strongly linked with the total count, while men's backs also highlighted an increased risk.
The researchers concluded: "We demonstrated that arm mole count of more than 11 is associated with a significant risk of having more than 100 moles, that is in itself a strong predictor of risk for melanoma."
Lead author Simone Ribero, of the department of twin research and genetic epidemiology, said the research was useful and relevant for GPs.
"The findings could have a significant impact for primary care, allowing GPs to more accurately estimate the total number of moles in a patient extremely quickly via an easily accessible body part. This would mean that more patients at risk of melanoma can be identified and monitored."
The research was published in the British Journal of Dermatology.
Dr Claire Knight, health information manager at Cancer Research UK, said: "This study suggests that the number of moles on our arms gives a good indication of how many moles we have on our bodies. This could be helpful because we know that people with lots of moles have a higher risk of melanoma.
"Other risk factors for melanoma include having red or fair hair, fair skin, light-coloured eyes or having been sunburnt in the past.
"But less than half of melanomas develop from existing moles. So it's important to know what's normal for your skin and to tell your doctor about any change in the size, shape, colour or feel of a mole or a normal patch of skin. And don't just look at your arms - melanoma can develop anywhere on the body, and is most common on the trunk in men and the legs in women."