The majority of heat is lost through the roof, windows and walls. And as heat rises, and the loft is usually a huge empty space, most of that probably goes through the roof.
So I guess there is some truth that if there is still snow on a roof the insulation must be good.
However I guess there are other factors, like if the loft has been converted into a room, and that has heating in it, then that will filter up through the roof as the loft area is warmer.
Also if a house is empty all day with no heating during the day the roof will still have snow as the house is colder.
But if somebody is in all day, with the heating on, then more heat is bound to go up to the roof.
I think snow on the roof is a general guide, but not the total indication.