How do I check if a URL pointing to a PDF (in a File column) actually returns the PDF?

[Viktor]

I’m working on an app that links a “unit” to a PDF document if said document exists. This is done with a “File” column and then a predetermined URL in the app formula. However, I need to be able to check whether a PDF is actually returned or whether the link points to nothing since the PDF might not exist when the unit is created in the app, and the choices a user can make depends on whether the PDF exists or not.
I’m assuming a simple ISBLANK() won’t do it, because I’m using an app formula, so the column is technically never blank even though a PDF isn’t retrieved.

Thanks in advance!