How can I find out if my niece has power of attorney?

IF the POA is recorded it would be in the County that is resided in. But POA's are not always recorded, in my State they are not. The purpose is to maintain confidentiality. Once recorded documents become public record. I would imagine many documents become public only if there is a court action.

Can you ask her?

Go to the courthouse and go to the county clerk's office. Tell the person who waits on you that you want to see the power of attorney papers for whom ever you are concerned about. Such as your mother or uncle or etc. They should be able to print out the papers in question, depending on which state or country where you live.