For peace of mind, you could review the owner's manual that corresponds with the crockpot (usually available on-line). I'm betting that most, if not all, models contain a thermal protector which shuts down the device if it reaches a certain temperature to avoid overheating. It probably also contains components with high flammability ratings that would not catch fire easily even if the crockpot somehow overheated. Overheating and the subsequent transfer of heat to flammable combustibles is the only way, that I can think of anyway, that a crockpot could conceivably be a fire hazard. However, keep in mind that crockpots were
designed for long-term cooking, so I don't imagine that using the product in conformance with its designed purpose would result in any harm. Also, crockpots are not terribly complex designs and they've been on the market for decades, so I would think that if there were any weak links in the design, they would have been worked out years ago.
Having said all that, I wouldn't use my crockpot in a way that wasn't recommended by the manufacturer, or hook it up to, say, 220 volts. But regular use as intended by the manufacturer shouldn't ever be an issue.
FWIW, I leave my crockpot on all day and don't worry about it.