Visit between 12-2pm and 5-7pm and ask for a Receipt

In the UK we have 2 periods which are known as Gapbuster Times. Basically between 12-2 and 5pm-7pm workers have to be extra careful because that is when gapbusters (mystery shoppers), who are paid, visit McDonald's restaurants and rate their experience. This then goes to the overall store rating and every store manager will do everything to ensure this rating is as positive as possible.

It is easy to spot a gapbuster because they always ask for a receipt (so they can get their food costs reimbursed by the company employing them). So, if you ask for a receipt in that time everyone will be immediately alert that you could be a potential gapbuster and they'll make sure to serve you the freshest burger and fries and give you priority over everyone else.

Source: I've worked at McDonald's for over a year in London