I suspect the claim "All beliefs are experimentally testable" is either vacuous or false. Our evidence for most of mathematics is deductive, not empirical. But it would be very strange to say that I don't have beliefs with substantive content about, say, the the Fundamental Theorem of Algebra.
You might say that mathematical investigation is a kind of experiment -- but in that case one wonders what causes for a belief aren't experiment. Is any evidence whatsoever 'experiment'?
I suspect the claim "All beliefs are experimentally testable" is either vacuous or false.
Clearly it's false. Plenty of human beliefs are non-testable, some even self-contradictory. LP did not claim anything of the sort.
Very brief recap: The logical positivists said "All truths are experimentally testable". Their critics responded: "If that's true, how did you experimentally test it? And if it's not true, who cares?" Which is a fair criticism. Logical positivism pretty much collapsed as a philosophical position. But it seems to me that a very slight rephrasing might have saved it: "All _beliefs_ are experimentally testable". For if the critic makes the same adjustment, asking "Is that a belief, and if so -" you can interrupt him and say, "No, that's not a belief, that's a definition of what it means to say 'I believe X'."
A definition is not true or false, it is useful or not useful. Why is this definition useful? Because it allows us to distinguish between two classes of declarative statements; the ones that are actual beliefs, and the ones that have the grammatical form of beliefs but are empty of meaningful belief-content.
It seems to me, then, that both the positivists and their critics fell into the trap of confusing 'belief' and 'truth', and that carefully making this distinction might have saved positivism from considerable undeserved mockery.