Try values really, really, really, really close to the number you're trying to find the limit on. You know, if you're trying to find the limit as x approaches zero, try 0.00000000001. Try negative 0.0000001. If you're trying to find the limit as x approaches four, try 4.0000001. Try 3.999999999999. And see what happens. But that's kind of the last ditch, the last ditch effort.