Just look at PEX-INFO.

unzip -qc foo.pex PEX-INFO | jq .

- it should be pretty obvious from there even if there are no ICs declared.

No ICs declared meaning "anything goes"?

Nack, look at the distributions in that case. Only Pythons that can load the root requirements transitively through that list of distributions. Given, you can't walk that graph by eye, but for most PEXes you can guess.