How is it priced?
Usually a minimum spend or a per head rate, sometimes with a travel charge. Ask which, because a minimum spend on a small guest list works out very differently from a per head price.
What do they need on site?
Access for the truck, level ground and often power, though many carry a generator. Say what the site is like in your request, since it is the thing most likely to rule a truck out.
What if it rains?
Trucks serve in the rain; your guests queue in it. Think about shelter for the queue rather than for the truck, and ask what they have done at similar sites.
Can they cover dietaries?
Most carry at least a vegetarian option and many do gluten free, but a single truck has a small menu by nature. Say the requirements up front so you are matched with one that can.