But if a Red Army soldier had never read any newspapers (perhaps being illiterate)

Soviet Union actively combatted illiteracy, especially in the Red Army. It was possible that a simple grunt from kolkhoz could be functionally illiterate, but he would still have contact with propaganda materials (rather than newspapers) and illustrations. He would most certainly see caricatures of Hitler and other Nazi leaders on propaganda posters.

or had seen any newsreels,

That's somewhat shaky, but not impossible, even though Soviets had no USO analogue. Films were rarely shown to soldiers recuperating, but it still happened.

Ideologically, most officers were expected to join the Party and NCOs and privates often followed suit, even if just to receive priority in food and vodka rations. Membership in the Party meant also political training and party meetings, where the goals of the Union were presented. During such sessions soldiers could see photos of Hitler.

And bear in mind that I'm talking about simple Ivans from kolkhoz. Even though rank-and-file soldiers might've been relatively uneducated, the structure of every military organization means that their superiors would be much more informed. When some groups specifically targetted high-ranking German leaders they were given some description obtained by Soviet intelligence.

would they have recognized Hitler on sight, and captured him? Could Hitler have slipped away unrecognized?

Where to? How? It's highly unlikely given what was going on in Berlin during the Soviet assault, and the personal convictions of Hitler.

I think not, a soldier wouldn have not been able to recognize Hitler. And even an officer neither. Nor could Hitler have escaped since there was just no way to run.