You'll find the 2013 kia optima crankshaft position sensor located behind the engine in the camshaft area, hidden a little behind the timing belt cover near the crank. this is a somewhat pricey part; the duralast brand one, with part number su9164, runs for around $ 74 at autozone. replacing it is pretty simple, as you just need to unbolt the single bolt mating it to the crankshaft, and disconnect the engine wiring harness, and then switch in the new one. indications that your crankshaft position sensor is going bad will be an engine that stalls or doesn't start at all, and when it is running, it'll do so poorly, and get really cruddy fuel mileage.
The 2013 kia optima crankshaft position sensor is behind the timing belt cover by crank.