This article is from the Real Ale FAQ, by with numerous contributions by Brett Laniosh others.
A: Look for the words "real ale" or "cask conditioned". For most people
the handpump is the sign of real ale but things aren't always that simple.
Some unscrupulous brewers and pubs use what looks like a handpump but is
just a keg dispenser. Sometimes real ale is served with added gas
rendering it 'non-real'. In certain parts of the UK (West Midlands, North
West) it is quite common for real ale to be dispensed with an electric
pump. If in doubt though, just ask. Bottled real ales are invariably
marked "Bottle Conditioned" and you may well be able to see sediment. The
following are NOT real and are Keg beers - all Nitro Keg such as Caffreys
and Guinness, Tins and most bottled beers. If it is served through a tiny
plastic tap attached to a fancy bar mounted advertising box or array of
taps on a brass frame then it is NOT real.