The word check is a noun, a verb, and an interjection. Example uses:
Noun: The check is in the mail.
Noun: There's a bed check at ten PM.
Verb: I'll check the price at both stores.
Verb: You can check off each book you've read.
Interjection: Did you finish your homework? Check!
Checked in is a phrasal verb it is the past tense of check in.
Mollusk is a noun. I know because it's a thing, and I checked the dictionary. (See related link below.)
part of speech
The part of speech for this particular word is a noun.
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "explicit" is an adjective.
The part of speech for "twirl" is a verb.
The part of speech for "unfamiliar" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "lecturer" is a noun.