It is easy to confirm (and you should confirm) that someone is a lawyer by checking the attorney registration data from the state where they purport to practice, usually available either online or with a quick phone call. If the person who claims to be a lawyer is not a lawyer then you are being defrauded.
If person claiming to be a lawyer is really a lawyer and says that they represent someone, you can generally trust that they are telling the truth about that representation. But, this doesn't mean that you can trust what the lawyer says about their client's rights to be accurate. You need your own lawyer to confirm that fact.
I have received a demand letter from a lawyer requesting information,
claiming to be working on behalf of the beneficiary.
Usually a beneficiary of a trust is entitled to certain information about the trust (which varies based upon the trust instrument and the law of the state in question) even though they can almost never actually compel the trustee to take action on their behalf.
This is so a beneficiary can determine if funds are being embezzled or improperly invested in which case the beneficiary can petition to have a new trustee appointed and can sue the trustee for breach of fiduciary duty with the damages paid to and held in trust by a successor trustee.
Also the rights of a beneficiary are often different in the case of a special needs trust established with funds from a third party (weaker in those cases if the trust instrument so provides) than they are in the case of a self-settled or court established special needs trust. Sometimes a trust will give a "trust protector" the enforcement rights usually afforded to a beneficiary (e.g. when a beneficiary is a minor or incompetent or incapacitated).
Also, FWIW, usually both a state attorney general, and the court with probate jurisdiction in the state where the trust is administered, and sometimes also the relevant DA, also have general supervisory power to protect beneficiaries of trusts who can't look out for themselves which presumptively includes most special needs trusts.