We time the songs we plan to do, add some time for introductions and chat, and estimate the set length long before the gig. You can also stick a little clock on your guitar or mike stand, or on top of your tuner... I've seen stick-on's as small as a quarter. Although it is good to ask the MC to signal you near the end of your planned material, so that you can go to your closing songs if you've run long, IMO it's less obtrusive to watch the time yourself, but discreetly.

~Susan