How to check if riding my custom entity?

I've got a custom vehicle mount and have sorted out controls for it that run off keybinds - you hit jump while riding, it jumps, all that.

What I'm having trouble with is how to check that the player is riding the RIGHT entity.  It's feels like it should be just a set of code blocks that go "if [get entity player is riding] = [this entity] then do [that bit]," but while I've found most of the theoretical parts of that, I can't actually find the [this entity] piece or find another way to get the "check that this is the right mount" to assemble into a set of blocks that actually click together.

I suspect this is dead simple and I'm missing something.  Anyone?