On the whole, food in Iran is hygienically safe and clean, and almost everywhere plastic gloves are used to handle food. On very hot summer days, avoid eating minced meat, grilled liver, or any other food that looks as if it may possibly have been standing around. Wash your vegetables and fruits, and if you have a particularly sensitive stomach, wash them with filtered water, or peel them.
When choosing a place for eating, see that it looks clean and is reasonably crowded, and also that the vendor looks clean and healthy.