In Ft Worth Costco, it was always a quick, cursory check. Mostly white and Latino, some black, but in general, most people here are straight arrow, law-abiding folks, with no reason to steal. There've been more issues with accidentally getting the wrong bag, common when they first came out with the idiotic carousal bag dispensers that mixed customers' items on occasion, than with catching shoplifters.
On the other hand, at the Costco in Long Beach (Signal Hill), the (usually black) receipt checker went through black people's bags with a lot more scrutiny. My bag, not so much. I think she was being realistic with her time and risk assessment, to be honest, but had she been white, it probably wouldn't have gone over so well.