High level only
Counterfeits exist, especially of popular coins and of small bars with famous stamps. The first defence is not a home laboratory. It is a counterparty you can still find in a year, a product that is common enough to be known, and a check of weight and dimensions against the published spec.
This is not a testing manual. It will not teach you to run acid, XRF, or ultrasound. A wrong test can damage a good piece. If a holding is large enough that a fake would matter, use a professional.
What a beginner can actually do
Weigh it. Measure it. Look at the edge, the reeding, the stamp, and whether the colour is trying too hard. Compare to a known genuine piece of the same product if you have one. None of that is conclusive. It is a filter for the obvious.
A price far below everyone else is information. Treat it as a warning, not a bargain.