If you have a specific food truck in mind their website should state where they are. But if something unexpected happens (run out of food, generator breaks, etc) and they have to close up shop early generally they only post those updates on Twitter.

There are also some aggregate websites for popular food trucks in a city. Here's the "Food Truck Fiesta" for Washington DC http://foodtruckfiesta.com/food-trucks-washington-dc/.

Have fun I was just in NYC last week and ate at a lot of food trucks. Good eats.