I always pull the chain taught then spin the wheel slowly checking for tight spots and if there is any binding slacken it off slightly until it spins freely.
If the chainwheel and sprocket are not too worn that should do it, if they are worn then check at the slackest spot hopefully you won't be able to move the chain more than 3/4 of an inch.
check the fixie FAQs here;
FixieFAQs

It depends how fast your cadence gets aswell, if you potter around slowly you can get away with a slack chain, around here there are some rolling and steep hills and descending at high rpm means I prefer to keep my chain tight as possible to avoid it geting thrown off.
Most of all mind your fingers always check from the outside of the chain don't get your fingers cut off by the sprocket