My strategy is to just ask someone who doesn't look too shady. People in New York usually have no problem helping out a lost soul, and tend to actually be quite proud of being able to help you. Here are some kinds of people NOT to ask, however:

Anyone who has already tried to offer you drugs
High-class suits (they have no time for puny mortals)
Large groups of asians with cameras (NYC noders know what I mean)
People who look more lost than you
People who are talking really loud to themselves

Other than these, I think you'll be okay. Have fun!