WiX usage is easy to check, just open an installer with Orca, and you'll see a bunch of WiX artifacts.

Checking whether it's their own compile or official binaries is also simple if they use an extension like the official Util one, which many do: It embeds a binary that implements the custom action, which is signed by FireGiant.

As for turning this into analytics / statistics, I imagine you could just download every MSI from winget and just check if they contain a FireGiant-signed extension dll.