baby poop question! (no pic)

So I just started my son on solids a few days ago. Started with bananas and he loves them, then I waited 3 days and tried sweet potatoes today and he loved those too! But I know if they get diarrhea there could be a possible allergy. But how do I know if it's diarrhea from being allergic? I mean his poop has always been diarrhea-like. But he just had a diaper full of greenish thicker diarrhea. So I don't know.