Not sure if it will be any help, but if it's a boat, aluminium from reputable suppliers will be labeled with the aluminium type.
Below the waterline is generally not insulated or painted and if you poke around you should find a stamp somewhere with the aluminium type. My boat has severalsuch stamps from the original producer of the aluminium.
Destructive testing must be possible as I have seen forensic examination of boat structures.Notably in about 1996 when there was a defective batch of 5086 produced with some very expensive court cases where the alloy was shown to be the wrong composition for 5086.
However I would imagine such testing would be very expensive.
Generally substituting a suitable marine
alloy is acceptable. The alloys are so close on the galvanic scale that there are no practical problems.
Most aluminim boats use different alloys for the extrusions (typically 6 series) and the plate (5 series) without any problems.