The Illearth War Gives a Strong Clue...
... to the readers, not to Thomas Covenant. Regarding whether the Land is real or not, the OP's question asks from two perspectives:
- Is his [Thomas Covenant's] uncertainty ever broken?
- Are we [the readers] ever told, either in the books or e.g. via interviews with the author, whether Covenant is hallucinating or actually being transported to another world?
The second book of the first trilogy, The Illearth War, reveals to the reader through the narration that the Land must be real. It does this through the details of Hile Troy's experiences in "Part II: The Warmark."
If the story were Thomas Covenant's dream relayed to the reader, the parallel time frame account of Hile Troy's experiences during Thomas Covenant's own experiences (given in "Part III: The Blood of the Earth") would not even be part of the story, for Covenant would not have been dreaming that while he was dreaming about his own role. This narrative shift of perspective gives a strong clue to the reader (not Covenant) that there are actual "events" occurring in parallel, indicating that the story is not existing in Covenant's mind.
This is strengthened by the fact that during the Part III narration, there is no awareness (by Covenant, Elena, or anyone) what is occurring as Part II had narrated.
Note that the argument here is not the fact that Hile Troy and the people of the Land:
claim that Hile Troy is from Covenant's world, as Covenant could be dreaming that aspect himself
It is purely that the narration of those events in parallel to the other events does not match the format of dreaming.
The argument here is based upon my own experience with dreams. When I have dreamed, I can at times shift perspective (and be someone other than myself, so Thomas Covenant could be dreaming as if he were Hile Troy for a time). Sometimes I can even see myself in third person, as might be the case at the end of Part II
at Covenant's reunion with Hile Troy and Lord Mhoram.
Additionally, when I dream, time sometimes "stands still," sometimes "accelerates" or "skips," but the dream always (for me) morphs along a linear path and never runs along two parallel streams simultaneously. So it is extremely unlikely that Covenant is:
- Dreaming these two sets of experiences in parallel to each other simultanesouly
- Dreaming these two sets of parallel experiences in sequence to one another (as narrated), with the latter experiences being a skip back in time during the dream
This, for me, is a strong (though not obvious) clue to the reader that the Land is real, even before that fact is made more explicit in the second trilogy per the point made in the other answer.