Oliver didn't know and it showed from his expression on his face. He didn't bother hiding his confusion behind a mask of lies, because he trusted Bart. You didn't get trust without getting trust. And Impulse never gave him a reason to second guess his belief in him. Finally he put away his thoughtful expression to explain his thoughts to his ally, "There's no Apokolips threat where I'm from." Could it be that others came from the future? He hadn't noticed that inconsistency with himself and Lois, but they both seemed to be avoiding the talk of the present when they were together.
He hadn't noticed it before, but this Bart was considerably taller than the one he knew. It caused him to doubt himself briefly. Did he trust in this person too much? Being in Red Oak as long as he had, did he learn to let his guard down? There weren't the same kind of threats here. No people playing mind games. No people finding ways to steal forms of the familiar to get close to you. At least not in his experience. But this Bart did seem as lost as he did. He even mentioned a change of costume. Maybe there was more to that future idea than he was thinking. As both a test to this person seeming to be Bart and a way to try to understand what was happening, he decided to offer up something from his own timeline, "How long has it been since we took out the Ridge Facility with Boy Scout?" For Oliver, not that much time had passed. He had since left Metropolis and been back once, only to return to the team again to continue their missions. Leaving both Clark and Lois behind him for a second time.