To formally announce one's arrival at a location or event (usually a hotel) to a proprietor or employee of that location or event in order to secure admission, accommodations, or other services.
To seek to verify (a claim or some data).
Can you check into his story? See if you can disprove it.
To announce on social media that one has come to a location.
She was downtown earlier, I saw that she checked into the Starbucks on Main Street.