Hi! Usually if a baby had thrush he will have white patches in the mouth/on the tongue, but sometimes there are no white patches and the babe still has thrush. Sometimes, too, a baby can get a yeast rash on their bottom if they have thrush in their mouth- it looks like a sunburn on their bum.
Can I ask what makes you think your babe has thrush? Are you having pain while you nurse? Is your child fussy at the breast and it's making you think that it's hurting to nurse? How old is your dc? If you could give me a little more info, I may be able to help you better!