The City of London trading standards service has issued beer mats, which can be held against the glass, to get a rough idea of the quantity of liquid they have been served.
It recommends drinkers ask bar staff for an immediate top up if their pint has a deficiency of more than 5%.
Should they receive a negative reaction from staff, or are too nervous to ask for a top up, then consumers can report the pub to Trading Standards.
The City of London trading standards service has written to all pubs in the area, reminding them of the importance to try and provide a full pint.
If it receives any evidence of pubs not meeting their legal obligations, they will visit to uphold the law.
Based on any complaints received as a result of this campaign, trading standards officers will be conducting test purchases across the City in the coming months and investigating pubs that continue to sell short measures.
According to City of London trading standards, UK law states draught beer must be sold by reference to quantity and normally in quantities of half a pint or multiples of a half pint.
It is one of the few imperial measures that are still allowed to be used following metrication some time ago.
The normal quantity of beer asked for in pubs across the City of London is a pint, which is measured into a pint glass that bears a stamp to show it is an accurate measure.
Most pubs use what are termed brim measures – this means they contain a pint when full to the top.
A few pubs may still use glasses that are called line measures, these are slightly over-sized glasses that have a line near the top to show the level of one pint.
Even fewer pubs may use what are called ‘beer measuring instruments’ that accurately dispense beer into a glass in multiples of half pint.
One of the problems with using a brim measure for the sale of beer is that it is virtually impossible to get a full pint of liquid due to the frothy head that is formed when the beer is dispensed into a glass.
Over the years, the generally accepted norm is that a pint containing a minimum of 95% liquid and 5% head is acceptable, according to the British Beer & Pub Association.
However, this is a legally complex area and trading standards said if drinkers ask for a pint, perhaps they should really expect to get a full pint of liquid.
As part of this project, a legal opinion has been obtained from experts at Gough Square Chambers. In an ideal world, all pubs would be required by law to use line measures, making it easier to get a full pint of liquid. However, no such legislation currently exists and pubs can legally use brim measures.
City of London trading standards manager Steve Playle said: “Consumers are well within their rights to make sure they get exactly what they have paid for.
“It is worth remembering that for a pint costing £5, a shortage of 5% is a 25p cost to the consumer. Drinkers are entirely within their rights to ask for a full pint of liquid if they wish.
“We are reminding people that it is perfectly OK to ask for a top-up while stressing that they should continue to drink responsibly.”