Avian pathogenic Escherichia coli (APEC) cause extra intestinal disease in chickens, turkeys, and other avian species via respiratory tract infection, giving rise to typical signs related to colibacillosis, such as: septicaemia, enteritis, granulomas, omphalitis, sinusitis, airsacculitis, arthritis/synovitis, peritonitis, pericarditis, perihepatitis, cellulitis, and swollen head syndrome. Disease caused by APEC strains results in significant financial losses to the poultry industry worldwide. This review provides essential information on the diagnosis of APEC. The information cited in this review will elucidate the various forms of APEC diagnosis while demonstrating the, lack of a precise diagnosis of this strain. The diagnoses mentioned herein reinforce the necessity to make progress toward a more accurate diagnosis of APEC to contribute to its reduction in poultry flocks.