It should read "it's" meaning it is useful to watch for internal warnings.
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Odessa DawnI am not sure if it is a contraction of it is or the possessive form of it. My choice is it is because I can replace it with it is. Am I correct, please?Yes, you are correct. Two other clues: there is no other word serving as the verb of your clause; there is no word to be described by the adjective useful.