Can I rely on smell to tell if rendered animal fat is bad? I've started storing rendered tallow (beef fat) in a glass jar on my kitchen counter at room temperature. The jair has a lid but is not air tight. 

Is it okay to use this fat for consumption as long as it does not smell bad? I'm worried about oxidation / rancidity.
You can smell and taste rancidity.  That is the primary way to identify it.  Rancid fat isn't necessarily unsafe, though there is some concern about rancid foods in our diet over time.  Given that we don't deal with health issues, that's about as far as I think I can go.