Guys, do you notice when another guy is checking a girl out?

The other day I was out with a friend, and we met a guy, whom I started talking to. When the conversation was the friend I was with (a guy friend) told me that the guy was super into me and it showed, and I should have done something about it. I didn't notice it at all, and I was wondering if guys notice when another guy is interested.
Guys, do you notice when another guy is checking a girl out?
Post Opinion