When selecting tempeh, look for a uniform, compact texture, and a slight off-white or beige color. It should bear a distinctive earthy aroma, which is from the fermentation. A few black spots on the surface aren't a big deal – they could simply be signs of healthy mold, which, again, derives from fermentation. That said, if the mold appears excessively dark, overly pungent, or slimy, consider avoiding that particular tempeh and opting for a fresher batch.