Answer: The subject of the sentence provided is "He".
Explanation: The subject of a sentence is the thing, person or animal being, doing or feeling something. A subject may include a head and its modifiers. Moreover, the simple predicate of a sentence makes reference to what the subject is, does or feels. In the sentence provided, the subject is "he", which is a personal pronoun and; therefore, refers to a person. Furthermore, the verb ("checks") indicates what "he" is doing.