Dan Peterson asked him directly about it:
So did Boyd Petersen (a son-in-law and author of his biography):I tried to talk to Hugh about some of those things, and he would not do it. Later on, maybe he would've spoken to me about it, but this was one [Adam-God] I was fairly junior in. I remember I had him once in the car, driving him to a fireside, I had driven him up and back from a fireside up past the point of the mountain. And I wanted to talk to him about Adam-God. I knew he had some opinions on it. He just said uh, "Well, I made a covenant with the Lord that I wouldn't discuss that with anybody, and I'm not going to." His wife was saying, "Oh come on Hugh, tell him something about what you think." "No." [Then he changed the subject.] He wasn't going to talk about it. And he didn't. So, I always regretted that, because I had him for fully two hours, by himself, trapped, and I couldn't get anything out of him. But on the other hand, I admire him for that. He was serious about it. There were some things he thought he knew and understood that he was not going to talk about. (Daniel C. Peterson, Mormon Stories 273, 32:00)
Putting two and two together makes me think he certainly did believe in the doctrine. He certainly was a big fan of Brother Brigham and was well acquainted with his theology. This is not a big deal of course, but interesting food for thought.I never heard him say, "I believe in the Adam-God doctrine," but it occurred to me when I was a student at BYU; I was fresh [off] from a mission, newly married, and really zealous about the Church. He had written all this stuff, apologetic, defending the faith, and I thought, why hasn't he addressed that issue [Adam-God]! So I asked him, I said, "why haven't you talked about that! The anti-Mormons get us on that all the time," and he said … "I don't talk about that." So, the implication to me was that he probably believed in it. He never confessed it. But he also said in several of his Brigham Young writings that he felt like Brigham Young was a better theologian than he's been credit for. So … my gut feeling is that he had very strong sympathies for that theology. (Boyd Petersen, Mormon Matters 187, 1:07:00)