My mother has dementia. Her late husband's daughter took over finances and won't give us any account information or details. She is not POA, just a signer on the checking account. How can we get details about all of her accounts and make sure they are being managed appropriately?
If you suspect Fraud it is time to take your evidence to APS and ask they open a case. If you have no evidence you should consider getting guardianship of your demented Mom if you wish to have it. If you do not wish to have it (YOU would then be the Fiduciary with all the burdens of that task in meticulous record keeping. You could also consider becoming the representative payee for your Mom's social security. Look on their site to see what all is involved; it's a lot.
You Mom apparently put her DIL on her accounts, or her name is there since the passing of her husband.
I think ultimately, if she is not open with you and you are worried, you are stuck with seeking guardianship.