thin ice

A sad story hit the news today: a mother and young son fell through the ice on a frozen pond in Kansas and drowned. Of course, they were visitors from California, where frozen ponds are unknown, and thus they might not have known anything about them, but that only increases the urgency of my question, one that's lurked in mind every time I've read a story including people venturing out on ice this way (for instance, American Gods): How do you know? How do you know the ice will be thick enough to hold you? Because if you're not entirely sure, it seems a rash thing to do. I've never been in a position to have the option, but my inclination to decline has reinforced itself.