Electrical outlet testers such as the one shown to the right are the fastest, easiest, and cheapest way to check if the outlet or extension cord has electricity and if it is wire correctly.  If can be very frustrating when an electrical device is not working and the cause is that there is no power for one reason or another.  Inserting the tester will provide one an immediate answer as to whether there is power or not.  It will also diagnose the wiring and indicate whether the wiring is correct.  The key on the tester show all possibilities from 2 ambers indicating that the power is correct to no lights indicating there is no power.  It will also show ground and whether the hot and neutral have been improperly wired.  In general the cost of an outlet tester can range from a couple dollars all the way to $20 or so dollars depending on extra features and capabilities.  Normally a simple basic outlet test is sufficient and I strongly recommend having one available