They visited 16 premises, of which nine were supermarkets, five were pubs and two were off-licences.
Eight out of the nine supermarkets consistently enforced their Challenge 21 policy, widely adopted by retailers such as Asda, by asking for ID. The off-licences fell foul of the scheme.
John Taylor, director of Trading Standards for Brent and Harrow, added: "Some retailers are misleading customers into thinking that they are going the extra mile in trying to prevent underage sales."
Separately, Ghafar Abdel, joint licensee of Costcutter Express in Manor Road, Stratford, has been fined £200 plus £750 costs for selling alcohol to a 15-year-old test purchaser in last year.