That is the way to test, and just that result is reported before. The Android build can usually be reimaged onto the SoC but the Firehose method has never been released or leaked as far as reported here.
But ofc you may have something different.
It is very difficult now. When liberating a phone with /e/OS I take a screenshot of all the software information before I start.
Here is an image from FP5 Vendor Security Patch Level (SPL) in Fairphone 5 - #11 by sophimae showing the two SPL. All images in the thread are non English.