Cephalotrocity , 6 hours ago

I don’t know for certain, but I think it is simply looking at what you do with your mouse. If the movement is erratic, imprecise, and delayed it goes ‘yeah, that is either a cat that got lucky which is close enough or a human’. The reason I think this is that I’ve failed same site’s checks if my mouse just happens to be hovering over the checkbox when the prompt appears. Retry, move the mouse, success.