Can I contest a will in which I was excluded?

Please accept my condolences on your loss. Assuming it was an actual, legal adoption (which it sounds like it was), then you are one of your father's heirs and yes, you have standing to challenge his Will. One issue is where he had his principal residence at the time of his death - since his Will will be offered for probate in that state, then that state's laws will control. Another potential issue is whether he prepared only a Will, or whether he took any steps designed to avoid probate and thereby make a challenge more difficult to bring, such as creating and funding a revocable trust. Another issue is how much of his assets may have passed outside of any probate estate or trust, such as assets he owned jointly with your stepmother (his wife) as joint tenants (or as tenants by the entirety, if he lived in a state which offers that form of ownership) and assets which went to a designated beneficiary under a beneficiary designation (which includes "Payable on Death" and "Transfer on Death" designations). If you want to challenge, you need to be prepared to move as quickly as possible, and you should not sign anything you may be asked to sign without first having it reviewed by your own attorney. Do NOT sign first and ask questions later - you can end up waiving your rights that way. Please also be aware that litigation can be very expensive, time-consuming, and emotionally and financially devastating, and that success is not at all guaranteed. Find a very good, experienced estate litigation attorney who works in the state where your father had his principal residence, and schedule a consultation. Be prepared to tell that attorney as much as you may know regarding the situation, assets, and documents. Be prepared to pay a retainer up front if you decide to move forward with a challenge, as most estate and trust litigation attorneys don't work on contingency (although some do so in certain circumstances). And best wishes to you.