How can i tell the difference between a rash caused by allergic reaction and rash caused by flesh eating bacteria?

Look at it: An allergy can cause a red sometimes raised rash- hives. The so called flesh eating bacteria is a nasty looking often with a bad odor lesion that can spread and get rotten looking. It can be painful. If you think you may have this go to your doctor asap.

Red versus dead: An allergic reaction is manifested by a raised red firm sometimes itchy but usually limited area whereas flesh eating bacteria create a rapidly spreading area of redness with fever and pain. The skin may blisters itch fluid or air bubbles or become grey/black.