On my last WAW-ORD flight, they were flagging passengers at the boarding gate who seemed to be exceeding the size limits. And forcing them to check one of their items (and paying a fee if it applied). The passengers basically had two items which could not fit under the seat-- when you're only allowed one item of such size. Also, when I landed at ORD and was waiting for my luggage at teh carousel, I was talking to a young female passenger who was caught at the gate, and was told that she was over the weight allowance. And she had to take some of the heavier items out of one of here bags (and leave them behind). The other option was for her to check the bag and pay the fee, since it would have been her second piece of checked baggage.

As for the check-in counter, I've only had them ask me to show them the items I was going to carry on board. They never weighed them. And only visually sized them.