Ask the testator herself if she has a will. No alternate, legal path leads to this knowledge given a living testator desirous of secrecy. The testator's attorney, doctor and banker are bound by confidentiality provisions from disclosing information about her private business to third parties. Her spouse and children may be honor bound, if not legally bound, to keep the fact secret. While you commit no crime if you attempt to pry information out of others, you risk the testator's displeasure. Question her directly if you need to know, but do not expect her to be pleased if your motivation is curiosity.
Confirm the existence of a will with a testator's executor after the executor's death. If the testator draws a will, she likely includes in that document the appointment of an executor, a person who shepherds her property through probate for eventual distribution to heirs. If the will fails to name an executor, the court appoints one. The executor -- who owes a duty of care to the heirs that precludes any self-dealing or dishonesty -- remains current of the will status. Call the executor's office to inquire about the will.
Travel to the court clerk's office in the court probating the will to learn the name of the executor or discover for yourself whether the testator left a will. Generally, the court in the county in which the deceased lived during the end of her life has probate jurisdiction. Provide the name and date of death of the deceased to the court clerk and ask whether probate has been filed. If so, ask to review the probate file. Court documents are public documents unless statutes provide otherwise, and the list of statutorily private court documents do not include probate documents. Review the probate file at the clerk's window or in a separate reviewing area. The file tells you not only whether the testator had a will, but the terms of the will and their validity.
Understand that no valid will existed if you find the decedent's estate distributed under the procedures of intestate succession. Intestate succession is a statutory means of distributing the property of a person who dies without a last testament. Exact procedures vary among jurisdictions but generally the property passes to blood kin of the deceased, regardless of whether the kin and the testator were close or even aware of each other's existence.