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Title
Google
Run the game, it often tells you in lower right or left corner.
Unless you have Galaxy configured to not update your games, it will always be the latest available from GOG, but on each game's info page in your library, it does indicate the GOG version number installed right next to the Play button. For games that have a changelog, the version number is a link to that log which may show the game's internal version number.
avatar
shaun.had: Title
You see your game's version on the right side of PLAY button.

You can also right-click on the game in the sidebar (or go to game view and click MORE) -> click "settings" -> disable auto-updating and you'll see info about your game's version and other available versions.
https://www.gogdb.org/ is the only known way as of 2023 without buying the game first. Shame on you GOG.