as long as who ever is doing the test has a unit for measuring tube steel as well as any plate areas, plus new unused specimen blanks at what ever hardness you believe the frame should be.
they are simple looking devices, but very accurate.
i guess the easiest validation would be test several areas on the end that has not been exposed to the fire, then test several areas that have been in the fire.
theoretically if unaffected, they should reflect close to the same based on proximity to welds etc.
still, at the end of the day, visual judgement may tell you what you need to know before any test happens.
