turn the cranks slowly, checking for slack as you go along. If the ring's not concentric, it'll be noticeably looser and tighter in different places. Sheldon has a section on chain tension in this article: http://www.sheldonbrown.com/fixed.html

Do you have enough room in the dropouts to get it tight or are you pulling the wheel back, hitting the end of the dropouts, and still not tight? Is it loosening as you tighten your axle nuts? Do you have track nuts? Normal cheapo flange nuts can cause the axle to walk forward as the rotating flange bites into the metal.