Is the lamb OK?

Hi everyone. I bought some lamb shanks Sunday. Today is Wednesday. I took them out of the refrigerator and they smelled pretty gruesome. I am sure they are OK but I could be forgiven for throwing them out.

Is there a way of knowing whether your meat produce is OK that is more sophisticated than smelling? I mean, I know if the smell makes you gag, it’s gone. But what if it just has a moderate farmyard-y kinda smell… is it OK then?

And is there a rule for the length of time you can keep meat before cooking it?