I think we'll have a pretty good sense of where the night is headed after the early polls close. Of course, as Mark has warned usrepeatedly, we want to be careful about reading too much into early exit polls before the weighting is adjusted to account for actual turnout and results. But here are some things to look for in a few states with early poll closings.
Indiana
Indiana is a state where Obama should presumably have benefited from the protracted nomination campaign and the massive organization he has build in that state. Thus, if turnout among young adults is going to increase markedly, it should be obvious here first. According to exit polls, in 2004, 14% of the Indiana electorate was between the ages of 18-29. We have to be careful with early exit poll figures since young voters may be more enthusiastic (and, thus, more likely to show up in early exit poll results), but if that figure goes up significantly in this election, then that is probably the first evidence we will have that young voters are turning out at higher rates in this election.
Virginia
Virginia may tell us more about this election than any other state. Not only does the map look very difficult for McCain if he loses Virginia (particularly if the networks can call it relatively quickly), but the demographics in Virginia can provide us with some useful insight into what may happen in other states. In 2004, exit polls indicated that African Americans made up 21% of the Virginia electorate. Will that figure improve in 2008 and, if so, by how much? What will the party id figures look like? In 2004, 39% of voters said they were Republicans compared to 35% who were Democrats. Democrats would like (and probably expect) to see those numbers flip in Virginia just as they are looking for party id gains in other high growth states like North Carolina, Colorado, and Nevada.
Georgia
Finally, returning to African American turnout, here is a remarkable statistic. In 2004, 834,331 African Americans voted in Georgia's presidential election. Already this year, 705,203 African Americans have voted early in that state. African Americans make up about 30% of registered voters in Georgia but in 2004 they comprised just 25% of the electorate. It appears as if we are well on our way to seeing a huge surge in African American turnout in Georgia, and when the polls close there at 7pm, we should have a pretty good sense of whether African Americans will make up 30% or more of the electorate in the state. If so, there is a reasonable chance that Obama can win Georgia and that a landslide may be in the offing. To do this, he needs to perform slightly better among whites than Kerry did. According to exit polls, Kerry won just 23% of the white vote in 2004; Obama would need 27-30% of the white vote to capitalize on the high turnout among blacks (or he would need Bob Barr to peel away a significant share of McCain's support). This is still a bit of a long shot, but Georgia has one of the first poll closings, so it will give us something to look for during the 7pm-8pm hour.