How to check if flatpak app is built from source or binary?

On Fedora Silverblue the recommended way to install applications is to use flatpak.
Lots of flatpaks are available on, both open source (such as Firefox) and proprietary (such as Skype). I know that there is a license field, but that does not indicate whether the flatpak is built from source (and thus can be rebuilt by me) or just a packaged binary of an open source project. One example is Anki, which is apparently just a repackaged version of the upstream binary and which seems to be difficult to build from source, even though it is AGPL licensed.
So, is there a way to see if a flatpak contains binary only content or is actually built from scratch?

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To my knowledge, the easiest way would be to check the .json/.yml I’m the flatpak git repos. They are very readable and would probably answer your question very fast.

Yes, I thought so too, but I am not sure what to look out for. Often there are multiple archives listed there and those are basically just a link to some .tar.gz, which can be anything, source or packed binary. So I would have to download and unpack all of those. Even finding some make command in there does not seem to be enough, as some archives could still be binary only.