New York City Sheriff Joseph Fucito said the sheriff's office (a separate entity from the NYPD) will conduct spot checks when out-of-state buses drop riders off at the curb, and will also check cars will out of state and New York licenses plates. Test-and-trace teams will also be on the ground to help direct people to testing sites while providing "education" about quarantine.
New York's statewide 14-day holiday quarantine mandates that travelers quarantine, or take a test showing they're negative. Violations of self-quarantine will be enforced, and may carry fines of $1,000 to $2,000, the mayor's office has said.
Around the US, few jurisdictions have actually enforced quarantine and social distancing rules, though people have been killed in fights spurred by mandatory mask requirements. Some governors, including Kristi Noem in South Dakota, have refused to make wearing masks and other social distancing measures mandatory.