How do you tell if he's actually off duty or just dicking around?
Source: http://nycitycab.com/NYC%20Taxi%20Guide.aspx#offduty
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.