Well, that's really easy for me, because your Mom says, "He's got the hiccups again."
How can she tell? That's a bit trickier. But, sometimes when you move it's a regular, rhythmic pulse that just so gently vibrates your Mom's tummy. I can feel it when I put my hand on her belly, and the difference between hiccups and your regular tumbles and rollovers and stretches is noticeable.
But, then again, maybe you're tapping your toes or snapping.
A few times your Mom has sworn that you've been getting hiccups in beat with the music that she's listening too. Who hiccups in beat? Perhaps you're just a natural born musician...both of your parents kinda have the music gene, afterall!
Or maybe we're just going crazy a little bit and looking for patterns and cognisance that isn't there yet....or is it?
The benefit to all this movement, besides that it demonstrates that you're active and healthy, is that you make it really easy to count your movements. We're supposed to keep track and make sure that you kick at least 10 times within an hour, twice a day.
With all this hiccupping, and moving and dancing, you move 10 times in 4 minutes! Even last night, as we lay in bed, I had my hand on your Mom's belly and felt you kicking like crazy as I fell asleep.

A lovely diagram of how big you are at 31 weeks. Not much room for toe-tapping.