Is there a way to tell if butter is spoiled?

Butter should have a firm, waxy body, good “spreadability,” and no moisture droplets. While color varies by region and type, it should be a uniform shade.
 
“Off” butter may appear pale, have blackish spots or mold, and be too soft or hard and difficult to spread. “Off” butter can smell stale, rancid, cheesy, fruity, malty, or decomposed.
 
Sources:
Wisconsin Cheese: USDA Butter Standards
Assistant Professor Franco Milani, Department of Food Science, University of Wisconsin
Journal of Dairy Science: “The Effect of Refrigerated and Frozen Storage on Butter Flavor and Texture” (2008)
The Microbiological Safety and Quality of Food by Barbara M. Lund and others, 2000.