I don't know much about google sheets, but as I understand it there is a last-modified timestamp? The simple solution is to store the last-modified timestamp in a Prefect variable, check against it and update it if you find it has been modified.
If you load a lot of google sheets, or a lot of other data sources, I would take a look at some more dedicated E/L framework. I use Meltano from within Prefect, and I see Meltano has a google-sheets tap (although I haven't used it).
https://hub.meltano.com/loaders/target-gsheet. This might take some more setup though, setting up a meltano project, and possibly use external state storage (S3, postgres or similar) if you use Docker.