It used to be easy to recognise.
The ‘punter’ pulls up to the kerb in his car, exchanges a few words with a girl in a long coat.
A quick handover of cash with shaking hands and the slam of a car door before the motor accelerates away in search of a suitably shadowy corner.
But like many industries, prostitution has moved with the times.
Street corners have been swapped for online bulletin boards.
Word of mouth whispered in seedy pubs has become five-star Trip Advisor-style reviews available 24/7 online.
Find your perfect girl, have your way then leave a rating for the next Joe.
Welcome to 21st century solicitation.
At any given time around 100 women advertise themselves as being for sale in Aberdeen on one well-used website. On the same given day you will find almost 30 in the Highlands.
The ladies, who age from 18 to 65, will tour the country stopping off in various cities for a few nights. In each place the call-girls will rent a private apartment or hotel room where they can take their punters.
They will offer a wide range of services, many catering for the most bizarre fetishes. And punters don’t even need to have a awkward conversation with the girls as a tick list at the side of their internet profiles details everything they will and won’t do.
These women are a breed apart from the stereotypical image of the mini-skirted prostitute leaning in a car window on a grubby harbour side street.
These are well-organised professional escorts charging £100-plus an hour for their services and holding down tour schedules that would bring the toughest rock band to their knees.
And in today’s modern world the anonymity of the internet allows them to continue living as mothers, wives and girlfriends while working undercover as escorts.
Around four years ago it was believed more than £1million a month was going out of the north-east on the black market through prostitution.
Girls can typically see four or five men a day and charge anything from £120 an hour to £1,200 for a full night.
Officers said that in reality the total could have been three or four times that amount. In an underground industry which depends on discretion it is impossible to make accurate estimates.
At the height of the oil and property boom Aberdeen became a magnet for sex workers who travelled from as far afield as South America and the Far East to capitalise on its high wages and steady turnover of clients with time on their hands and money to burn.
And last night Detective Chief Inspector Stuart Houston said it was almost impossible to tell whether the decline in the oil industry has put a dent in the market.