Is there a way to tell if margarine is spoiled?

Margarine is vulnerable to heat, oxygen, and moisture, so “off” margarine may melt or harden and smell a bit like soap, paint, or microwave popcorn.
 
Fresh margarine should have a buttery flavor and be smooth, firm, homogeneous and “spreadable,” with no droplets of moisture.
 
Sources:
USDA: Specifications for Vegetable Oil Margarine
Assistant Professor Franco Milani, Department of Food Science, University of Wisconsin
 
Fats and Oils by Richard D. O'Brien, 2003.