Generally, men should have about 8-10% body fat while women should have about 15-18% to have visible abs. You can read more about this here.
Anyway, here is an interesting chart about the optimal body fat percentages.
But how you can measure your body fat percentage? The video below will show you that, you can find other methods under the video.
How to Measure Body Fat - Health.com
Whether your priority is looking good in a bathing suit or living to see 100, there's one piece of information that might stand in the way. Body composition, the measure of fat mass to lean tissue (including bone, muscle, ...Bodybuilding.com - How To Measure Your Body Fat
Pinch yourself. No, you're not dreaming. You're measuring your own body fat with the help of a friend. Follow these instructions, keep practicing, and you'll have all the info you need to track your progress!