At the top it'll say "You own this game", including what the price was and how long ago it was, although some places my change their price later on, making you have to buy the newer version, or just straight up make another page you have to buy.
Other than that, this one doesn't do that, so feel free to buy it if you want, just know the free version was the last build created before the current one for this area, but when it becomes finished, do know you'll need to buy the full version, like this for example: Paccsu by Paccsu (itch.io)