Other questions will be asked to help rule out those other medical conditions especially infection or food allergy. The selection of tests may depend on whether or not the child is currently having an acute episode or is currently well and has recovered from the event.
To learn what conditions may mimic FPIES symptoms, it may be helpful to review this medical journal article: Differential Diagnosis of Food Protein Induced Enterocolitis Syndrome.
Fiocchi A, Claps A, Dahdah L, Brindisi G, Dionisi-Vici C, Martelli A. Differential diagnosis of food protein-induced enterocolitis syndrome. Curr Opin Allergy Clin Immunol. 2014 Jun;14(3):246-54. doi: 10.1097/ACI.0000000000000057. PMID: 24739227; PMCID: PMC4011629.