I think in general, US airlines don't tend to worry about carry-ons, especially on the mainline planes. We once had a planeside bag (the kind the regional jets do where you pick your bag up at the plane, not baggage claim) that must have been filled with bricks. Felt like it was over 50 pounds. We did a gate check for that one instead because of its weight, but that was really crazy.

On the other hand, the European airlines routinely check the weight of carry-on bags at check-in. I've had that happen a couple times in Spain on a paid ticket, but thankfully the first checked bag would be free on those airlines. I also had to have my bag's weight checked on CX FRA-HKG and HKG-SFO, which led to me having to check those bags as well.