How do I know if baby has allergy to formula?

[Gages'&Karlannas' Mommy]

our daughter was born premature and is on Neosure. Latley while feeding her she grunts and scrunches up her body as if trying to poop. She then starts screaming and crying. I thought she was trying to go to the bathroom but a lady said it looks like she might have a allergy to her formula, that babies do that if they have an allergy. It takes me and hour to feed her 3 oz she eats alittle then grunts and screams so i stop and burp her than try again. We do this over and over till her bottle is gone. I know shes hungry but its as if shes in pain when I feed her. I already called my doctor who hasnt gotton back to me yet but I just wanted to know if mabey this is a sign she has a allergy to Neosure?