Find a real gamut

[kolak]

How to find a real gamut used for given file?
If you grade to eg. P3 and export this as Rec.2020 then chromaticity diagram will show that colors are not limited to P3, but Rec.2020.
Is there a way to find original gamut in files which went through a color space conversion?

2nd question.
Is it possible to tell that given file which hash headers saying Rec.2020 in reality is P3 based (just file headers are wrong)?
I assume this is easier when file has Rec.2020 bars? What if bars were Rec.2020 but P3 limited during export ?

[richardpl]

The only way that I know, is capturing known colors (via colorchart or other means - colorbars) and comparing with known correct reference. If file does not label colors in its video frames in some means, you are mostly blind.

[wswartzendruber]

You could theoretically put it into XYZ to find out, but you would need to know the format the source bitstream is in.

[kolak]

It should be always Rec.2020, but some bad exports will be P3 just with Rec.2020 tags. I'm looking for an objective way to finding out which case I'm dealing with. To make things worse (I think) even if it's real Rec.2020 export it will have gamut clipped at P3. It's basically P3 within Rec.2020 container.