Clues that a Sharepoint site is customized using SFPx

josh_vts Do you have access to tenant app catalog or site collection app catalog?

Check this if it helps: Experiment – Find out where SPFx Web Parts are being used in Modern SharePoint sites  

If you want to check for specific site page - if you have access to edit the page, you can identify which web parts are used in the page. Here is a list of SharePoint out of the box web parts: Using web parts on SharePoint pages . If you see any other web part apart from this list, it means the web part is custom web part.

Please click Mark as Best Response & Like if my post helped you to solve your issue. This will help others to find the correct solution easily. It also closes the item. If the post was useful in other ways, please consider giving it Like.