Hi @Pablo Glomby ,
Using SharePoint Rest Api or CSOM, we cannot check if the site is used in Teams. As you said, we can only get the WebTemplate value. But that's not enough.
As far as I know, when a site is used in Teams, the assosiated M365 group will have Teams connected. You can identify go to admin center> Groups>there you can see Teams status and symbol to identify that SharePoint site is used in Microsoft Teams. See image below for your reference.
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