The manufacturers of GFCIs and UL, who do the testing and listing, both accept the test button *on the GFCI* as the valid way to test it. Here's an interesting scenario for you - You use your handy 3 prong circuit tester with GFCI test button to test a receptacle that is *not* a GFCI type. You do this because this particular receptacle is next to the sink in the master bath on the third floor, and you know it's required to be protected. The receptacle responds by going dead. Now what? You spend however long it takes to find the actual GFCI on that circuit and reset it. What if it's behind a permanently installed cabinet? What if it's behind the owner's hutch filled with 400lbs of his families' fine antique china? What if, also, there is a freezer in the garage, buried under much of the owner's belongings and out of sight, that's filled with expensive meat, or a wine cellar that must be kept at a low temperature to protect thousands of dollars worth of fine wine? You're screwed. If I can't find GFCI where there should be one, in spite of having tripped any visible ones and noting any GFCI breakers in the box, I report it as such and suggest an electrician check it out for safety.