You don't know for sure. But we have some clues:

1. The man has clear facial injuries consistent with a blast whether it was a mine, mortar, artillery shell, or whatever. He's been hit in the face and body with shrapnel. That's not makeup. The swelling of his eyelids and raccoon eyes are consistent with that as well, reflecting the shockwave hitting the soft tissues of the head.
2. His eyes tell us a lot. I don't see panic or fear. Given the water in his hands and the empty bottles above his head, it looks more like the fear hit when he was injured in combat, and he is now calm because there's someone here who's going to see to him. This is consistent with his story. Perhaps it could be faked.
3. He is worried about wanting to satisfying his interrogator, but because the Russian man shows some compassion, we see him maintain eye contact and often offer clarifying information to try and ensure the man understands what he is saying. This is the sign of a man who wants to tell the truth and be properly understood as cooperating. Again, you could script it, but the body language matches the behavior and the choice of words (assuming the translation is good. I recognize enough Russian words to match some it)
4. The man is speaking Russian perfectly, but that's common especially in eastern Ukraine. The confusion with Mariy is a pronunciation issue across the language barrier, again, to be expected, and both men handle is as if that's expected sometimes and move on.
5. This is far from the only video like this. Telegram is full of such videos.

There are some other things:

1. The guy's story is a common one: grabbed on the street, minimal training, shoved into combat and told to attack, so he does. He gets injured and caught and doesn't know much because he's a conscript.
2. He doesn't know any of his commanders' real names. He named one man, but knew everyone else by callsigns. This is something I've seen a lot. Ukraine seems to have adopted this as a way to disguise their identities and provide a modicum of security against Russia's excellent human and signal intelligence.
3. The villages/cities he mentioned were accurate in that they're known locations for troops training and Ukrainian formations. It's a story that's plausible based on what the war mappers have shown us is happening on the front lines.
4. The Russian interrogators gives us some clues too. He's clearly trying to empathize with the man. Many such videos show the same. They're acutely aware of what the Ukrainians are told will happen if they're caught - that the Russians will torture them. We get that same BS narrative out of the MSM. In fact, they've been specifically ordered to treat these men well. This is a "brother war" for the Russians and they intend to live with the Ukrainians when this is over. They don't want to be seen as monsters or war criminals and this interrogator specifically points out that he's trying to treat this man well, even as you can hear the artillery going off in the background. They're still close to the front.

On the whole, sure you could say it was a good fake, but I'm inclined to believe this is genuine. There have simply been too many like it to think it's a one-off, IMO.