The answer to 'how to test GFCI outlet' is to regularly test your GFCI outlets to ensure they are functioning properly. GFCI (ground fault circuit interrupter) outlets are designed to protect against electrical shock hazards by quickly cutting off power if a ground fault is detected.
To test a GFCI outlet, first, plug a GFCI tester into the outlet. This specialized tester will have a 'test' button that you can press. When you press the test button, it should cause the GFCI to trip, cutting power to the outlet. If the GFCI does not trip, then the outlet is not functioning properly and should be replaced by a licensed electrician.