Don't ask them ANYTHING. If they are approaching you, it IS NOT legitimate.

Illegal to offer unlicenced taxi services in NYC but there are plenty of gypsy cabs that flout the law in areas of the city where yellow cabs do not want to cruise.  It is like the illegal aliens; technically deportable but not enforced.  When you say 'what should I ask' do you mean how much?  Generally you agree a price verbally with the driver before you get in and then pay that amount at the end.  Tips are not necessary.  If you are nervous, use Uber.  Price will be about the same.