How can I verify the expression of dialogflow?

I have created a pizza BOT in dialogflow. The scenario is like ..

The robot said: Hi, what do you want.

The user said: I want a pizza.
If the user says I want watermelon or I love pizza, then dialogflow should respond to the same error message and ask the same question. When getting an effective response from the user, the robot should prompt the second

The robot said: What kind of pizza do you want.

The user said: I want a mushroom (any) Pizza.
If the user provides some junk data, I want icecream or I want good pizza, and the robot responds to an error, you should ask the same question. I have trained the robot with the intention, but the problem is to verify the user input. How can I make it in dialogflow?

How can I verify the expression of dialogflow?