You can tell fudge has gone bad if it appears dry or cracked. If you notice an oily film on the top, the fudge spoils. The cookie dough fudge should never be mushy or slimy, and if it appears to be, then it isn’t edible at this point. Typically, if the fudge has gone bad, you won’t notice a smell like other foods. Don’t risk eating it if it has gone bad. Just toss it and make a new batch.