I go up to them, and I ask them.
And they usually like to spend any amount of time explaining, if they see that you're someone who might be able to understand the answer. And that's to a large degree just attitude. People are usually really happy to talk about things they know a lot about, not just to sound smart, but because they really like to think about it, and you get to think about it when you hear yourself talking... I really enjoy ranting, and listening to rants. I learned a lot of CS by listening to hour-long rants at the cafeteria. Some nerdy person just piping their /dev/urandom into your visual cortex.
Knowing what to want to know seems much harder.