I briefly worked in an arcade a few years back, so was changing a lot of £1 coins. 1 in 30 sounds about right, it was amazing how many fakes we'd see.

Like others have said, the lettering around the edge is always the biggest clue, but also the head and tails stampings will sometimes be off centre, and misaligned to each other.

The fake ones also make the 'wrong' noise when you drop them on a hard surface. It's hard to describe, but really noticeable if you compare it to a real coin.