I went for the look up for 30 secs option, but that not stricly true...
If I'm playing then I'll let someone else take the initiative or let them keep the ball rolling, and then grab the rulebook whilst the focus isn't on me - that way I don't slow games down.
If I'm GMing then it's more complicated, and I'd be more inclined to wing it, but if someone launches into a long-winded description or something, then I'll grab the chance with open arms and reach for the rules. If I'm using my laptop to GM, then it takes literally seconds to find the right file in the SRD and to check the right line - a lot quicker than turning pages.