How do you test for food allergies in a 7 month old?

My darling has what I’ve read online is an allergy ring.
A red ring around her anus to be precise.
I’m a liitle confused as the pediatrician says it’s unlikely an allergy yet everywhere online says it is from allergies.
I’m wanting to check the foods I’ve been giving her just to rule out any allergies.
I’ve read about the 4 day rule – my question :
Do you feed say on Monday avocado then wait till Thursday and change food – meaning only Monday does she get avocado or do I feed that same food for 4 days straight.
This is going to be difficult as I’ve introduced her to so many different foods!
I should add her poops are quite solid – a bit like big rabbit poops so something is definitely irritating her!

Thanks.