How old is your baby? Is she on formula? Since when did she first gets the rashes?
I'm not a medical professional but I have a 19-month-old who has ezcema and food allergies. MY experience is if she gets rashes on the flexures, it's likely to be ezcema flare-ups. If it's on the back or tummy and a lot, it's food allergy. IF it's on the arms and hive-like, it could be food allergy or contact allergy. Try change her clothing and wipe the arms with cooked water and see if it goes away. Food allergy doesnt go away after you cleaned it.
OK, actually if you wanna check what your baby's allergic to (food), you'd better do a skin prick/blood test. But if you can also do it on your own as a start.
(1) Try to remember when the rashes started to appear and what did u give her to eat that's new around that time. Allergic response can occur straight away to about 48 hours after the food.
(2) IF you suspect something, say eggwhite. Try rubbing a tiny bit of RAW eggwhite on her arm (find a spot where there is no rash) and check for rashes within 20 minutes. If there is no response, try rubbing some on the chin. This is the most sensitive part of the skin and will more likely to get a response. IF there is still no response, she's likely to have NO ALLERGY or allergy but not severe to the food.
(3) Remember to use raw food because cooked food change the protein structure of the food.
(4) Try to avoid the food that your baby's allergic to and see if there is an improvement to the skin. If there is still no improvement, you might need to start feed your baby with basic food like rice, pork and green veges. Add one item each time and see if she gets a response.
With my experience, to find out what the allergy good is takes a lot of time and patience. Please let me know if you need more info, i'm willing to share more.