Hi KGV,
Well, for me, either I refer to my Postage Stamp Catalogs (i.e. Scott, SG ...etc) & check the date inscribed in the sheet. If I could not locate it (
after doing an extensive search), I give a shot to whatever "references" I have relating to the Cinderellas for the said country.
Otherwise, I surf the net & try to find any info. Sometimes, I refer to one of my expert friends/dealers (
although they are handful!).
My doubts arise when the Min. Sheet has
NO "face value" (
either on the imprinted postage stamp or the sheet itself).
Perf. or Imperf. sheet has nothing to do with being definitive/commemorative issue or just a Cinderella.

I'm sure that my other fellow collectors will have better ideas & methods than what I do ...
Hope it helps!

Cheers