Checking for cabin bag size is pretty rare on domestic jet flights. Only once I have seen them make everyone put their bags through the size/weight thing.

A few times for full flights I have seen them pull side people with bags that look big and have those bags measured. Some bags were under and some were over. The ones that were too big had their bags tagged at the gate desk and put in the hold.

But for most domestic jet flights there is no checking, although if you had a bag that looked way over the limit I would expect you would be stopped.