City drinkers should feel confident asking for top-ups on short measure pints, officials have said.
Trading Standards officers from City of London say a pint should mean a pint of liquid and drinkers should request a top-up if they want one.
Beer mats have been produced to help consumers check if a pint is short, and drinkers are being encouraged to report a short pint.
Reporting unhelpful reactions when asking for a top-up is also encouraged.
Trading Standards Officers are due to carry out test purchases across London in the coming months and will investigate premises that continue to sell short measures.
The Weights and Measures Act 1985 says a pint of beer should be exactly that.
But the industry body, the British Beer And Pub Association, says a pint should contain a minimum of 95% liquid and 5% froth.
Steve Playle, Trading Standards manager at the City of London Corporation, said: "Consumers are well within their rights to make sure they get exactly what they've paid for.
"It's 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."