Hi, my name is*****'m a certified appraiser, and I'd be happy to help.
This is a particularly difficult issue, as forged gold bars will often have a very thick coating of real gold, enough to pass a basic nitric acid test, density test, or scratch test.
Perth mint gold bars are one of the most forged items, so you are right to be very cautious.
One way to check is the thickness. While the counterfeit bars are usually correct in length and width and weight, they are often thicker than the norm, in order to accommodate the lighter-weight metal inside but still have the right weight. Anything off by even a millimeter in any direction, or partial gram in weight, is probably fake.
But really, unless you are buying from an authorized distributor, the only way to be sure is with an XRF metal analyzer. And then you need every bar analyzed, lest the seller give you one good one to test in a batch of dummies.
These bars are simply very risky to buy or trade on the secondary market as they are too easy to successfully fake.
Please let me know if I can help further.