How do I find out if my brother has power of attorney on my mother?

I used an attorney to find out and was successful.

I would start by asking him.
As you may know, the POA isn't free/doesn't have to/shouldn't discuss POA actions and duties with others. However, you are right to be concerned whether or not your mom HAS a POA if you feel she NEEDS a POA.
If asking gets you nowhere, then do answer a few of the following questions and hopefully we can guide you a bit.
1. How old is mom and does she live alone; if not, with whom does mom live?
2. Is mom currently competent under the law and making her own decisions?
3. Who currently is paying mom's bills?
4. What concerns do you have? Are you fearful there is elder abuse? If so, have you contacted APS to ask for wellness check, and to check if mom's finances are safe and being documented and recorded?

Welcome to the forum. Know that in order to help you we must have information.
Wishing you the best.

Looks like if there were two posts, tge other one was deleted.

You have 2 posts of the same topic going on, but you have more detail in the other post so I'll report this one as the duplicate.

If you suspect elder abuse, contact an attorney. You should ask your mother if she has given POA to your brother. There is really no way to find out.