At an auction, I purchased a 400 gallon diesel fuel tank for $150. I
thought that it was empty, but turns out that it was not, it is
approximately 1/2 full, so maybe 200 gallons.

It contains dyed fuel, so we cannot use it in our semi truck legally.

I do not need as much off-road diesel fuel, and I need to know if it
is winter fuel or summer fuel. I could use some winter fuel, but not
summer fuel, this is why I am asking.

So, how can I tell them apart? Maybe put some in a bottle and in a
freezer? Any signs to watch out for?

I will sell most of it, the question is now much.

i