Yeah, I agree with muck.
The more muscle you put on, then less accurate body fat % will be.
There are stupid ways of checking using your hight and weight etc... but they are no good.
Basicly -
10% - if you have a firm 6pack
15% - if you can see your six pack slightly.
20% - if your stomach is not flat, but not big.
25% - if your stomach hangs over your trousers.
Thats not accurate at all, but its all you need to know really.