That's a good point, that nowadays people who call themselves "prog" or "liberal" really aren't either (in the classical sense of "progressive", like a Bull Mooser), but are in fact Stalinists in drag.
The proof of thesis would be a line by line comparison of that person's known public opinions and affinities, to a) the 1934 COMINTERN platform (or some similar definitive docutment -- there are three or four that could serve) or b) the 1952 or 1948 Democratic platform (in 1948, the pseudo-"prog" Stalinists were all off at a rump convention, so the real Democrats were running the Democratic Party).
Proof of the pudding.
Programmatic comparo, point by point, and then score each personality or celeb or newspaper against the two, and assign them to either the CPUSA in Drag, or the old liberal crowd, objectively. Hubert Humphrey and Adlai Stevenson would doubtless be liberals, as would be, say, John Lindsay, former mayor of New York.
Yes, and if you want to see a liberal stumble all over themselves, ask the question -
“so, if democrats/progressives/liberals aren’t communists, how would their beliefs be different if they were?”