For "in-hand" PM:
Stick with small bars and coins from reputable dealers. IF something does not appear correct then STOP the deal.
Buy a scale and micrometer -- see weights and measurements of typical coins on this blog site or other internet coin sites. Weigh and measure coins. Use acid tests if you must "ruin" the coin.
Beside coins (which are almost impossible to fake the size and weight of) , purchase small bars - 1oz or less, because they small bars can be measured and weighed and they are more difficult to drill and fill or plate. Older scratched small bars can be acid tested with less fear of any diminish cost.