How to Know if Baby Has Thrush?

My sister in law came over last night and told me it looks like my 3 week old has thrush because there were “blisters on her lips...” I looked at the inside of her mouth and lips and didn’t see anything at all, her lips just look a little chapped and they have since she was born, they looked the exact same last week right before some dead skin came off. I looked at the roof of her mouth too because my sister said it looked white but the only white I saw is on her gums and it’s not patchy or anything, and it looks the exact same on both sides. Should I still make her an appointment? Is there anything that for sure means she has thrush? I don’t know what to look for