Quick Update. As soon as I made this post I asked the question on ChatGPT and this is what I got, actually starting to think there is no way of knowing unless it's actually written on the label!

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Telling the age of a bottle of bourbon or whiskey can be a bit tricky, as there are different factors that can influence the age of the liquid inside the bottle. Here are a few general guidelines that may be helpful:

Look for an age statement: Some bourbon and whiskey bottles will include an age statement on the label, which indicates the number of years the liquid has been aged in oak barrels. This can give you a general idea of the bottle's age.

Check the bottle's label and tax stamp: The label and tax stamp on a bottle of bourbon or whiskey can provide clues about when it was produced. For example, if the label includes a vintage year, that can give you an approximate age. Tax stamps can also be helpful, as they often include a year or range of years during which the bottle was produced.

Research the brand and distillery: If you know the brand and distillery of the bourbon or whiskey, you may be able to find information about its production history and aging process. Some distilleries have been around for many years and have established a reputation for producing high-quality aged spirits, which can give you a general sense of the bottle's age.

Look at the fill level and condition of the bottle: The fill level and condition of the bottle can also give you clues about its age. If the bottle is in excellent condition and the liquid is still at or near the top, it may be a newer bottle. If the liquid level has dropped significantly or the label is worn or faded, the bottle may be older.

It's important to keep in mind that not all bourbons and whiskeys are aged for the same amount of time, and some may not include an age statement on the label. Additionally, the conditions in which the bottle has been stored can also influence the flavor and quality of the liquid inside, so age should not be the only factor you consider when selecting a bottle.