To recognize constituents we should use constituency test. There are numerous constituency tests applied to English sentences
• replacement by a single word => if a string of words in the sentence can be substituted with a single word (e.g. a pronoun), it is a constituent of the sentence
• omission => if we can omit a string of words from the sentence and this does not affect the grammaticality of the sentence, this string is a constituent
(attention! There are some constituents that cannot be removed from the sentence e.g. subject)
• question test => answers to “WH” questions (who, which, what, why, where, when, whose, how) are constituents