There is no industry standard as to how brewers date their beers, though most use a “bottled on” format. That style indicates when a particular beer was canned, rather than when it is best by. Often, that date can be found on the bottom of cans, on the side of bottles, or on the case itself.
The value of a vintage beer cans can run the spectrum of virtually nothing to upwards of $25,000. The three factors that impact a cans worth are: rarity, desirability and quality. Any can missing one or two of these factors will suffer in value.
On cans, most date codes are printed on the bottom—and in the case of Oskar Blues, there's even a special phrase or witty message. If you can't find a code printed on the label or anywhere else on the bottle/can, the last resort is to check the carrier or case packaging.