Modern technology uses X-ray fluorescence. You can literally just point a handheld device at a block and find out what it's made out of (including Gold). You can do the same with jewelry and determine if it's made out of solid gold, gold plate, etc.

Here's a product video of one example: http://www.youtube.com/watch?v=1Fu8uLBdgcE

Here's another neat video demo: http://www.youtube.com/watch?v=rYLxmRsFIWE

Edit: Also, the Curiosity rover has an X-ray fluorescence instrument as part of its "CheMin" spectrometer which it uses for analyzing the composition of Martian minerals.