My 10 year old daughter wants to know how she would know if she has gas like has to fart? preferably a girls answer?

Make sure she understands that all humans fart; it's completely normal. Usually it can be felt before it happens and temporarily suppressed or delayed. Some foods produce more gas than others; she can learn to avoid some that are a problem. If it seems excessive or with diarrhea, a doctor can evaluate for special conditions like lactose intolerance. Good luck to her.