How can you tell that your baby is allergic to formula? I already consult to my pediatrics about it, explaining the symptoms and he said that my baby should drink soy formula instead. That I did.

The symptoms he has (that I noticed) are vomiting, a bit of rash on his neck and white tongue. He also easily catches cold, despite the fact I nursed him exclusively for the first three months.

The thing is, my friends comment that maybe I should just change the brand and not go directly using soy formula.
Any suggestions?