There are 4 main ways of checking whether a pound coin is fake.

1) D/M Match

Does the Date and Motto match the Design.
Printable table courtesy of the Royal Mint -

2) Is the Motto clear and centred.

3) Are both sides of the coin the right rotation.

(With the Tails side facing you, flip the coin horizontally on its axis)

4) Are the coins Speficications correct.

Official figures are:
Diameter: 22.5mm, Weight: 9.5g & Thickness: 3.15mm
(Personally I only check the weight)

Last but not least,

Sometimes a coin matches all the above, but is still possibly fake.
(It could look too shiny, have uneven wear etc)

Ones that pass the above 4 Steps, but don't FEEL right,
generally go in my 'Possible Fake' pile.

PS All Coins in the images above are fake !

Copyright The Fake Pound Coin Database.