Identifying Phosphor Omitted GB Pre Decimal Stamps

I've seen a number of stamps listed as "phosphor omitted".

1962 - 67 commemoratives had phosphor and non phosphor versions so how would you tell the difference between a phosphor omitted variety and a stamp which wasn't supposed to have phosphor in the first place?

"Phoshor omitted" has no trace of phosphor where it should have been. So, stamps issued during the 1960s trial period that have phosphor omitted cannot be distinguished from ordinary stamps, unless a different press was used to print the two varieties.

There might be two other ways:
1. the stamp is on cover or card cancelled in one of the trial areas. I doubt it will work as it is possible someone used a stamp bought elsewhere.
2. The manner in which the phosphor was applied would leave a trace of the cylinder impression when there is no trace of any phosphor on the front.