After the motorcycle/truck scene, we see John Connor touching the T-101 model to grasp the fact that Arnold really is a terminator. After touching him, it seems like he grips something between is index and thumb. Is this a metal shard of sorts or is he just pushing his fingers together to register what he just touched. I am guessing the black that I saw between these fingers was just oil from his motorcycle?
The shooting script explicitly states that he's touching the Terminator's blood:
JOHN : No way!
John touches Terminator's skin. Then the blood on his jacket. His mind overloads as the reality of it hits him.
JOHN : Holy shit... you're really real! I mean... whoah!
JOHN (Continued) You're, uh... like a machine underneath, right... but sort of alive outside?
TERMINATOR : I'm a cybernetic organism. Living tissue over a metal endoskeleton.
And the Frakes novelisation gives us a little more info;
“No way!” John blurted out, but he was already convinced. Still, John touched Terminator’s skin. It felt warm, but somehow ... different. Then the blood on his jacket. It looked very real. And now he let the realization sink in, and his mind overloaded as the reality of it hit him.
“Holy shit ... you’re really real! I mean ... whoa!
You’re, uh ... like a machine underneath, right ... but sort of alive outside?”
The cyborg answered as if reading a technical manual.
“I’m a cybernetic organism. Living tissue over a metal endoskeleton.”