How do I judge?
It helps if you can be more specific and not use loaded terms like 'intelligence'. If it was a single metric placements would not be so complex.
Here are some things I look for: intrinsic interest/self-starting, logical reasoning, high-level or conceptual thought processes, abstract or mathematical mental models (e.g. non-procedural thinking), open-mindedness/lateral thinking/connecting non-adjacent knowns, algebraic/symbolic representations and manipulation.
That doesn't even make up half though. A larger part is more about how they actually go about working day-to-day. Are they easy to talk/disagree with? Show continued interest in learning or content to apply what they know? Can work on longer projects to completion? Do the necessary boring stuff.
A good team needs mostly the latter qualities and a few members that can sometimes be the 'cooks' in the restaurant.
what made you think that?
Is this hard? Or are you asking how to judge when you don't have access to all the metrics normally used by admissions committees for this purpose?