You will wish to review the probate court records in the town or city where your grandfather resided when he passed away. The will is a public record, so you can obtain a copy for a small cost. Good luck to you.
This information is presented as a public service. It should not be construed to be formal legal advice nor considered to be the formation of a lawyer/client relationship. I am licensed in Connecticut and New York and my answers are based upon the law in those jurisdictions. My answer to any specific question would likely be different if I were to review a client's file and have the opportunity to interview the client. Accordingly, I strongly urge you to retain an attorney in your jurisdiction with respect to any legal matter.
I assume you have already asked his closest relatives and they can't or won't answer you. The most obvious place to look for a will or a record of a probate proceeding is in the county records of the county where your grandfather lived before his death. Even if no probate has been opened, your grandfather may have filed a will with the registry of the court during his lifetime, and any person with the original will is obligated to file it with the court shortly after the death. Note that in Washington, you can open a probate in any county. So if you don't find a record in his county, you should keep looking. It is also quite possible that your grandfather had no will and no probate has been opened. If you know the name of his attorney, you could also start asking there.
This answer provides general legal information and should not be construed as legal advice to be applied to any specific factual situation. It is not intended to create and does not create an attorney-client relationship. The attorney writing this post is licensed in Texas and Washington only and the laws of your jurisdiction may differ.