You could shut her in a pen for two days & nights-- if she is laying, she should lay within that length of time, and then you'd know for sure.
I've had some hens that did, and some that did not.
I think there were more who did not, than the ones that did.
Does water freeze? In the daytime?
If not, I'm positive it is not too cold for her to lay eggs.
I grew up in Alaska. I'd go through the snow to the chicken pen, replace the frozen water water with fresh water twice a day, collect the frozen eggs... (Yes, that freezing water was inside the chicken coop. The chickens themselves were fine, but frozen eggs can crack open and then they leak when they thaw.)
Some hens will not lay when the days are short. Since short days and cold weather usually go together, people think (wrongly) that the cold is at fault. A light bulb for a few hours each morning or evening is much more useful than a heater when someone wants winter eggs. Of course hens will not lay if they are starving or dehydrated, so you need to be sure they have food and water (not ice), but that is the only way cold seems to affect laying.