Hard to tell by the odor alone.If you really want to be sure, spoiled fox urine taste a little like road killed skunk.Seriously though, when I did a lot of trapping, I'd buy urine by the gallon.Pure red fox urine smells like a combination of red fox gland scent and ammonia.The older the urine gets the stronger the ammonia smell becomes.From a trapping perspective, urines that I held over till the following year were just as effective on fox and yotes as the fresh stuff.If the urine actually has a spoiled smell, quite likely it was contaminated in the collection pens with bits of food dropping into the collection trays.If the trays arent cleaned regularly, the urine will run over spoiled food and will absorb some of the smells.Pure unadulterated urin from meat fed red foxes is primo, and usually commands the best prices.Stay away from the mass produced cover scent blends, and buy it in bulk from a reputable trapping supply company.If you really want to boost the covering/attraction capabilities of the urine, try adding 1 oz of pure red fox gland lure to a quart of urine.Really boosts that foxy smell.