How can I find out who has POA over my mom?

I'm posting what you wrote in your profile because I think it's helpful in this discussion:

"I am a concerned about my mother. My sis will not put on her doctors list, and she only tells us bit and pieces of her health. And that's why my brother's and sisters want to see if she has the power of attorney."

I think a lawyer will say it's nobody's business who your mom's PoA is, but as JoAnn29 commented, this little bit of transparency goes a long way in keeping a unified family effort for an LO's sake.

She may not want to put you on her doctor's list because there should only be one point person (the medical PoA) when the LO is incapacitated. It could sow confusion if there is more than 1 person contacting the doctors directly. You will need to ask your sister to please be forthcoming with important medical info so that sibs are not in the dark. People seem to imagine all sorts of inaccurate, nefarious intentions when there's a lack of transparency. I hope your sister will loosen up the flow of information if she is the PoA.

This is a question we see often and I am going to have to remember to ask my lawyer when we get our POAs done.

I see no problem in someone needing to prove they hold POA. They have to show it to banks and doctors and anyone they need to talk to concerning the persons finances. So why can't they be made to show it to their siblings. All they need to show is the header with the Lawyer, than the LO saying they have assigned so and so then the LOs signature. Also signatures of the witnesses and notary.
How else are you going to know. My brothers were aware I was assigned. I lived in the same town. But if they had wanted proof I see no reason not to give it to them.

Now, the POA does not need to reveal the finances or medical info. My brothers never really asked. But I knew my Mom wouldn't mind them knowing so I would say there is enough for a year in the AL. With her medical, I kept them in the loop. They are her children.

Diah1960, the best way to find out to whom your mom has given POA, if anyone, is to ask your mom. Your profile says she has Alzheimer's dementia, so she might not remember correctly or remember at all, but I would still start by asking her. You can also ask your sister if she has POA and if she does, you can ask to see it so you'll know exactly what it says. That said, there is no legal obligation for your mom or your sister to answer your question or show you any documents.

If you suspect your mom is endangered in any way (e.g. by your sister's refusal to "put her on doctor's list"), then you can try to spend more time with both of them to see for yourself if anything is amiss. If that's not possible (e.g. because your sister is controlling and/or isolating your mom), then you can call adult protective services (APS) to request a welfare check for your mom. Best wishes.