If they are captured on the battlefield in uniform engaged in combat activities, then there is no problem. You just swear to the time manner and place of his capture.
There you go again.
They have NO UNIFORM.
What if they are captured off the battlefield transfering weapons, or making plans, or setting up an ambush, or any other thousand ways the enemy is captured?
I think you have no concept of what is really happening.
Well, then you just state the facts. You obviously captured them for some reason, in some place, doing something that you regarded as an act of war. You so state. Nothing more.
Your problem is that you are trying to make this too hard. It isn't hard. Just hold a neutral hearing, with adequate representation, and confine the guy as a prisoner of war or as an unlawful combatant.
By the way, under the Geneva convention, the individual does not lose all rights because you classified him has an illegal combatant. In fact, you are required to treat him as a POW until you can bring evidence to prove that he was an unlawful combatant. It is a higher standard of proof, not a lower one.