Ordering food when you have dietary restrictions

What is the right way to ask over-the-counter-food selling people about the food? I keep having the problem where I ask things (like, what is in the food, for instance) and they interpret this as me ordering it and start making it for me. I want to be respectful and not a jerk to the people, but I can’t just let this go, because the reason I’m asking is that whether or not I can eat the food depends on the answer.
realsocialskills said:
 
I’m not sure, because I have a lot of trouble talking to people who are selling me things.
 
I suspect that part of the problem might be tone, or not using clear enough words.
 
If that’s the problem, then stating the problem first and then asking about the food might help:
  • “I’m a vegetarian. Does the lentil soup have any meat in it?”
  • “I can’t have gluten. Can you tell me which dishes are gluten-free?”
  • “I’m allergic to mushrooms. Does the chicken sandwich have mushrooms in the sauce?”
  • “I don’t like olives. Does the bean salad have olives in it?”

In terms of not being a jerk, it helps to say thank you when they answer the question, and when they give you edible food. 

It’s ok to interrupt if they’re in the process of making possibly-inedible food, but I don’t really know how to do it effectively.

Does anyone else know good ways to handle this? How do you get information at food counters that will tell you whether or not you can eat the food?