a pair of words, as pin and bin, or bet and bed, differing only by one sound in the same position in each word, especially when such a pair is taken as evidence for the existence of a phonemic contrast between the two sounds.
The teacher can check whether they can distinguish these two sounds accurately with a minimal pair exercise.
British Dictionary definitions for minimal pair
(linguistics) a pair of speech elements in a given language differing in only one respect and thus serving to identify minimum units such as phonemes, morphemes, etc. For example, tin and din constitute a minimal pair in English