It is important, as a parent, when dealing with this issue to proceed cautiously. Especially if your daughter is older, say in middle school or a teenager, it is important to balance both her safety and also not make her feel like she has no autonomy. It is also important to be affirming and supportive and not make her feel threatened, as if she is being bullied at school she probably feels this way there.
One strategy that can work, depending on the age of your daughter, can be to ask her open ended questions, such as to ask her if she has anything that is bothering her or that she needs to talk about. It can also help to remind her explicitly that you are supportive and that if there is anything she needs you will be happy to help or to listen.
At the same time, discretely, it is not a bad idea to make some inquiries with your daughter's school principal or main teachers. They may be able to give you some insights into her classroom performance and peer relations and let you know if they have any concerns or observations that correlated with your suspicions.