The lights. A NYC cab's lights look like this: |O|Number|O| Where "O" is "Off Duty" in smaller letters. If just the Number is lit, they're available, on the

The lights. A NYC cab's lights look like this: |O|Number|O| Where "O" is "Off Duty" in smaller letters. If just the Number is lit, they're available, on the clock. If Just the Os are lit, they have a fare. If both are lit, they're available, with destination discretion. If none are lit, they're not working at all.