Membership of CAMRA confers many benefits. But the greatest of these is the Short Measures Card. This is a credit-card sized piece of laminated card, for use in pubs.
It has a grid whose columns are likely beer prices from £1 to £3 or so. You line the top of the pint-glass up with the price you paid, and read off the level of beer you were given. The resulting number will show you how much money the publican has overcharged you by not quite giving you a full pint.