Checking Gold To See If It's Authentic

The chances of getting anything bad from those two sources is as close to zero as imaginable.

Checking the weight and diameter is a good first step - weeds out the total garbage. The standard values are widely published. If the item is a couple grams light, it ain't genuine.

XRF is used, but that only checks the surface.

There are several models of inductive-resistance testers that can differentiate among fineness and even different alloys.

Beyond that, you would need to do a specific gravity test and check the item with ultrasound (this detects gold plating over tungsten core, etc.

That's about it for NON DESTRUCTIVE testing.

Obviously the gold standard (pun intended) is a little hard on the item: Melt it, stir well, cast a bar and then assay a sample using either traditional methods or a mass spectrometer.

(If you want to see an example of this, hunt up the last episode of the 1st season of "Gold Rush"

There are a 1 oz gold testing kit to check weight, width and depth for all major gold bullion, worth it if you are a major buyer of gold.

https://goldcoinbalance.com/

At the coin shop I work at, we use one of these to test all the gold we buy...

http://www.sigmametalytics.com

As a side note, it can also test silver.