How can you tell if a rash is caused by an allergic reaction?

My doctor told me to start "rotating" things such as for a few days use a different lotion, then a soap, then a laundry detergent, etc. Also foods, medications and fabrics may need to be rotated.
Your best bet really is to go get allergy tested.
You will probably have little red bumps all over your body an if its something you ate or got into your blood stream into small cuts, you will have them on your tongue also.
It is an allergic reaction if it is sudden and keeps spreading over hours. Then, you look at what you have eaten, worn or done differently over the past few days. Taking benedryl can help alleviate some of the reaction, but once you've been exposed to a "toxin" it takes a few days to pass it. Oatmeal baths and Aveeno anti-itch creams help. OTC 1% cortisone cream can also help, but not for long term. Avoid spray on "--caines." Major culprits for me are soaps with bleach, makeup, lotions, new clothing/sheets/towels and new meds. Try putting a cup of white vinegar into your final rinse cycle to get the soap residue out.