Remember that a bar/ button with a tungsten core would pass the specific gravity test.
...nor an XRF test, since the x-rays can't penetrate very far through gold. If the gold wrapper is more than 1mm thick, a regular handheld metal-checking XRF won't see deep enough to spot the tungsten. Even the top-model highest-power XRF analysers can't see through more than 2mm of gold. You'd need gamma rays from a nuclear reactor, or take it to CERN and put it in their particle collider in order to generate radiation powerful enough to see straight through it. I don't know if making a tungsten-cored fake button ingot would be easier or harder than making a fake "chocolate bar" ingot.
You can check for non-tungsten base-metal fakes easily enough with the scale - assuming it's a 1 ounce button, does it actually weigh exactly one ounce, and if it does, is it significantly physically larger than another ounce of gold you happen to have lying around? A specific gravity rig can also give you peace of mind; even a rough and ready rig can tell the difference between an ounce of gold and an ounce of gilt brass.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis