Hyundai Repair: 2002 Hyundai Accent, crankshaft position sensor, hyundai accent

Question
I purchased a used 2002 Hyuandai Accent for my 16 yr. old daughter.  The problem we have it with it occassionaly not starting.  The first time it happened, it was checked and not getting any spark.  Our family mechanic advised it is not the fuel pump.  He advised it is probably a sensor.  Any way to confirm this?  We have been advised to put it on a computer.  This is hard to do because usually within 15-20 minutes of it not starting, it will start.  This normally happens more frequent right after putting additional gasoline in it.  ???????????

Answer
Usually, if the problem occurs after fueling, the purge valve is stuck open.  But this won't make the vehicle have no spark.  Similarly, a defective fuel pump won't cause a no spark condition.

Lack of spark is most frequently caused by a defective crankshaft position sensor.  Often, this will leave a diagnostic trouble code, even if the check engine lamp has not yet come on.  

How was it determined that there was no spark?