Intelligibility testing works by having listeners distinguish acoustically close consonant pairs

Evidence

Subjective measures of intelligibility are often based on the use of lists of words that rhyme, differing only in their initial consonant. In a diagnostic rhyme test (DRT), listeners fill in the leading consonants on listening to the speech, and often the possible consonants will be indicated. In a modified rhyme test (MRT), each test consists of a pair-wise comparison of acoustically close initial consonants such as feel—veal, bowl—dole, fought—thought, pot—tot. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2399

Related

• Rhyme tests pinpoint exactly which consonant distinctions a speech system fails. (similarity 0.93)
• Nonsense-syllable tests measure intelligibility by stripping away language-level prediction (similarity 0.79)
• Speech audiometry tests intelligibility by varying signal level and speech pace (similarity 0.78)