I hired an electrician to install a GFCI outlet outdoors. I need this to power some pumps in an aquaponics system, so I need to guarantee it works, so kids don't get hurt while playing in the water. How can I verify the electrician installed it correctly and the system is safe?
The TEST button is a pretty darn authentic test
The TEST function on a GFCI is required to create a current imbalance in the sensing toroid, exactly mimicking the action of a real ground fault. (See UL 943, clause 5.15.3 for the details.) So, you can be confident that a GFCI that trips when you push TEST will actually trip, whereas the TEST button on external testers may fail due to a receptacle being miswired in certain ways (such as Hot/Ground reverse) or a simple lack of equipment grounding provisions to the receptacle. Note that NEC 406.4(D)(2) permits the use of ground-fault circuit interrupter protection as a safety retrofit when equipment grounding is unavailable.
You can buy a receptacle tester for $10 or less, this will allow you to test the GFCI function and check that the wiring is correct. You plug it in to the outlet in question, and the indicator lights will show whether your wiring is correct or in a few common incorrect modes. The button also tests the GFCI, which should trip the outlet and force you to reset it. There's a million brands, but they all look something like this:
A outlet tester may not identify all possible mis-wirings, but it should be enough to give you some peace of mind that the job was done correctly, and that the GFCI is functional.