When food comes in a box, a jar, or any other type of package, in the US there is an ingredient list on the package. I know reading every label can be tedious but please read them all.
The first rule in gluten free cooking is to avoid any item where the label indicates the food is processed in a plant that also processes wheat or indicates that the product may contain wheat.
Many times it is less clear whether or not a product contains gluten. One way to clarify whether is gluten in an item is to contact your guest with Celiac ask for their experienced input.
Another option is to do the research yourself. Food manufacturers maintain websites with nutritional information and allergen lists, if it is still unclear, most companies have a contact email or phone number. I usually get a prompt and clear response when I ask a company about their product being Celiac safe. There are also websites and apps for your phone that have full lists of ingredients and brands to look out for. I use an app on my smart phone “Is That Gluten Free?” which is updated regularly and clearly lists ingredients as well as a product’s gluten free status with easy links to more explanation. The app costs about $5 and it is a great wealth of information and a time saver.
Sometimes the label is clearly marked gluten free, which in the US means that it has been tested and conforms with standards requiring that the food contain no more than a tiny trace of gluten. If your guest with Celiac disease has agreed to come to your house for dinner and have you cook for them, then I expect that they can tolerate this tiny trace of gluten allowed by FDA standards.