Rummaging around in the fridge,I found a half-open pack of Danish Blue.It doesn’ smell odd,but it is covered in blue furry bits and mould.Since it was like that to start with,how do I know when it’s gone yeuhhh?
Hmmmm, while I can’t answer your question, personally I wouldn’t be taking any risks with cheeses that contain live molds, like Danish Blue cheese and Camembert.
No “Best before date” ???
BB’s not for a few more days.But it’s been floating round half-opened in a fridge 
My sainted mother always said that blue cheese smells like ammonia when it goes bad.
Danish blue shouldn’t be covered in furry mould at all, in fact no blue cheese should. The mould should be in ‘seams’ throughout the cheese but if there is furry mould on the surface (is it like the furry mould you get on old bread?) then it has gone off. Saying that, I would just cut the surface mould off and eat the interior.
MMMMMMMMMMMMmmmmmmmould! 
Blue mold and a strong foul odor are the signs that blue cheese has gone bad.
I thought the odor at least would have suggested it was fresh 
I guess it’d be cheaper to run out and buy more cheese rather than taking the chance and eating it, IMHO.
Willass is correct. Blue cheeses have the mold-creating enzyme injected into their structure, and no mold forms (or should form) on the outer surface during ‘manufacture’. So if there’s mold on the outside, it’s gone bad.
When it complains that it can’t reach the maraschino cherries in the bottom of the jar and asks for some tootpicks so it can finish them off. That’s bad.
Then there was the comedian (Steven Wright, perhaps?) who wondered how you could tell when sour cream went bad: “Did it turn…sweet?”
It’s a mold culture (Penicillium roqueforti, not a mold-creating enzyme.
The mold continues to grow after purchase.
If it’s gotten all blue and fuzzy on the outside, you’ve waited a bit long to eat it, but absent bad smells, or spots of different colors, it should be OK to eat.
If you have a bit of mold to spare, toss it into a pint of sweet cream and let it incubate for a few days. Yumm! Consult a French cookbook for the recipe.
Stinky shrapnel.