If you took an original hydrometer reading for Original Gravity, then take a reading for the Final Gravity, then for a typical example:
OG = 1.045
FG = 1.008
The equation would look like this:
((1.05 x (1.045 – 1.008))/1.008) / 0.79 = 0.487
So, this brew would be about 4.9% alcohol.
If you didn't take an original reading then drink five pints and apply the following scale:
Weak: Wow that's tasty
Medium: Didn't realise how spunky next doors' Mrs actually is.
Strong: Waaahhh, why did Princess Diana have to die so young and leave us to grieve forever .... waaaahhhhh
edit: Glen's formula is less stuffing around