The GFCI testers out there are good, but the only approved method for testing GFCIs is via the manufacturer's instructions, which only call for the push-the-test-button method.
What can you use to validate whether the external tester's is testing correctly? What do you do if the test show a bad outlet? Did you know that most of the testers have a time limit for use? You cannot use them on every outlet in a room unless you let the tester "time out" before the next use. Kind of defeats it's purpose, don't you think?
Use the push button, and verify it with a light.
IMOI, Rick Miell
