There are many ways to make the creation of a substance or an item difficult today. Complexity can be worked in at various levels, from microscopic to macroscopic; it can be introduced in controlled amounts (fine print on a bill or passport, difficult to forge) or in uncontrollable ones (forging all the details on the entire Mona Lisa takes massive amounts of work; breaking a suitably complex code makes a supercomputer burn out). Creating very small amounts of something may be difficult (fine jewelry; nanoparticles of gold in exquisite red glass), or creating very large amounts may be (you can make gold out of lead, but in order to even see the amount of substance you created, you have to work for millennia using current methods).
If latinum were trivial or merely difficult to replicate, no ban would protect it from forgery. Some sort of complexity is necessarily part and parcel of the value of latinum. Is it "random", "natural" chemical complexity, the sort that makes it difficult to replicate tricyanate or certain biological materials? Is it a chemical code built into the substance, so that while one can replicate latinum, one can't replicate latinum of worth without knowing the code? Is there even a difference?
We know Quark can recognize the smaller GPL-based means of exchange by the sound they make, and by their taste or consistency (i.e. the biting test). This tells us basically nothing, though, as all these features could be there on a superficial, macroscopic level - Quark wouldn't be bothered to go too deep for mere slips. Quark has not been witnessed verifying the value of entire bricks by such crude means; perhaps these greater-value items have more complex codes, only readable by technological means?
Also, that gold is worthless without the latinum within doesn't necessarily mean the latinum would be of worth in itself... Quark found the stuff in "Who Mourns for Morn?" and was delighted, but then again, he also possessed the gold casings at that point; perhaps it was only the combination that made him (temporarily) rich?