You can start by checking with the bank that your mother was a customer at. However, they may not be able to tell you any information about the trust due to confidentiality issues. The only other way to find out about the trust is to contact your mother's lawyer, which she would have had to hired one to work out the legalities of the trust. They would be more apt to give you information than a bank would. You could also file an injunction on the trust with your local courts and your sister would then have to give up the information about the trust.