How do I tell if DD has a yeast Infection?

Is she potty trained? Look for rashy-looking blistery spots on her labia, esp. if she is in diapers.
Check her panties or diaper for abnormal vaginal discharge. If she has a yeast infection she may have some that is thick and yellowish. Her pants could also smell yeasty- like bread yeast.

Those are the things I'd look for.

I've also read that the rash can look very pimply. If I suspect that my daughter has one, I usually start using Lotrimin AF at each diaper change. If it's a yeast infection, it helps, if it's not...it's not an worse than using normal diaper cream.

dd has had a few if them and she's had slightly raised, very red patches that have an edge around them. we use a thin layer of lotrimin af and cover that w/ a thick layer of burt's bees diaper rash cream - for both vaginal and rectal yeast infections. they usually go away in 2-3 days. use the cream at every diaper change and for 24 hrs after it looks like it's gone.