It depends on how cold. The ground insulates tremendously, so when in-ground, even if its extremely cold air temp, say -20, -30 F, its much, much warmer in-ground even an inch or so down.

When they're above ground, they can tolerate moderate freezes, probably ok for sure down to about 25 F, and possibly as cold as maybe the mid teens. But if allowed to get much colder, they're toast.

I know this the hard way, from losing bulbs that were above-ground when the weather was too cold.

It probably also depends on the species, some may take colder temps than others.

You probably won't know for sure unless you either cut a few open and see if they appear firm and solid, or the tissue is soft, watersoaked. Or, if you plant them and see if they start to grow roots and shoots.

I'd give them the benefit of the doubt and at least see if they're alive. Maybe there were buried in "stuff" in the car and didn't freeze as hard as you think. I've left fruit and other groceries in the car in winter, buried under "stuff", and they have survived far colder temps than I thought they would.

The key thing is that the freeze happens SLOWLY enough that the cold triggers the bulbs to produce the enzymes that allow it to survive freezing. If it's warm and they freeze suddenly, they'll die. That's also true for any perennial plant including shrubs and trees and it's why you wrap young trees in the winter to keep the sun from heating the trunk too much.

You will know if the bulbs are dead or not in two ways: smell and squeeze. Healthy bulbs will be firm. Bulbs that have been destroyed by frost will be squishy and probably leak liquid when you squeeze them. Also, healthy bulbs will smell like soil or won't smell. Bulbs that are destroyed by frost will probably have started to rot and because they are very energy-dense their decomposition is anaerobic and smell like a spoiled potato.

If they are firm and not rotting, go ahead and plant them, they're probably fine. In my experience a dead bulb is pretty obvious.