Physical constants are only assumed constant

Claim

Physical constants may not have been constant.

In point of fact, the notion that physical constants really are, well, constant, has been tested in a number of ways. For example, observations of distant astronomical phenomena show they are operating under the same physical laws as they are today, indicating there has been no significant change to physical constants. So far, the results of all such tests indicate that any variance in physical constants cannot amount to more than a fraction of a percent per billion years or so; therefore, the constancy of physical constants is not an assumption; it's a empirically supported concept.
This kind of argument can be made about any theory. There's always a chance of incorrectness. It's equivalent to saying "your proof still isn't absolute, so I'm not convinced". The problem with this is that absolute proof never exists in the sciences.
If we assume without evidence that physical constants are variable, it raises questions as to why they might vary. This runs counter to the principle of scientific parsimony - never propose unnecessary complications.
If we assume that physical constants can and do vary, then nothing in science can be trusted.
This is an old chestnut of pseudoscientists which deserves a wiki site all its own, as it would require terabytes (equaling roughly a master's degree in physics) to fully refute it. Since the claimant has simply made a vacuous and unelaborated statement, it deserves a no less vacuous and unelaborated answer: Wrong.
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