have a friend take your camera into the bar and set the ISO to 800 then use the cameras meter to decide if there is enough light 1 to 1/2 stop over what you would like. If not, set the camera as if there was 1600 film in it and test again, and so on until you know what film you'll need to use. giving yourself a half stop to full stop extra space means more likely than not you'll be fine, and even have room to stop down the aperture a little more.
of course this may require a quick lesson on your camera and its features, as well as using a light meter.