How cold does it have to be before diesel gels?

So now I have a question about cold weather and diesel fuel and since the OP question was about that with a generator thrown in my question is:

How do you know when it got cold enough for the fuel to spoil the fuel filter?? What do I need to look at?

I am in SE Kansas now it has been subfreezing now for about 5 to 6 days with temps down to 4 last night and 7 a couple nights ago and a lot of teens. My fuel filter is hanging down from the Freightliner Chassis pretty much exposed to the cold I did fill the tank, 100 gallon, in October when I got here. The filter does have the plastic water separator on the bottom.

JimR