My son is showing some slightly little changes, but when I ask him if he does he tells me no.
Use your gamma-ray vision goggles and check their bladder fill level. If somebody lifted the set that came in your parenting handbook (quite common considering their value on the black market) you're just going to have to wing it. Use your intuition. You probably know your child's mannerisms better than they do, so use your best judgment. But also expect to be wrong and treat it as if you're not sure. When it gets to the point that you're pretty sure, call a timeout and ask them a favor. Request they go through the motions and spend the time to go—even if they don't think they need to.
By doing this you'll prevent accidents and your child will learn to better understand when they need to go. Judging that takes practice and most won't stop to think about it and analyze whether they could go given the chance. So give them the chance to learn that they can often go even when they didn't have to go this second or have an accident.
But also learn to trust them. As they figure it out better and start giving you more reliable yes/no answers start trusting their response and slowly move the onus to them to be right or wrong. But still point out useful break points or places until they learn the drill.