I workout on a regular basis.i lost 31 kg over the last year.but i don't know if i have skin fat over my belly.how can i know whether its skin or fat?

Probably both: When you lose a lot of weight, your fat shrinks faster than your skin can keep up. This often leaves a lot of excess skin, which does tighten up in time, depending on age, and many other factors. Plastic surgery can be considered if there is a lot of excess skin, but give your body a chance to catch up first, and then try to get comfortable in your skin.