On Tuesday these are the things to look for...
Conventional wisdom is the low turnout benefits Brown and high turnout helps Coakley. What is really important is who turns out and where that turnout is coming from. If there is high turnout in the towns in the Merrimack Valley, the 495 belt (watch Marlborough and Shrewsbury) and the South Shore/Cape, it may be a good night for Brown. If turnout is high in the urban centers, inner suburbs, and liberal communities west of Boston (Newton, Concord, Brookline, etc) Coakley may be on the road to victory.
If Chelmsford reports results and Brown is not leading by a substantial margin, he is likely to have a bad night. Conversely, if Coakley is winning Somerville, but by only ten points, she will likely need to work on the concession speech. The anecdotal evidence based on absentee ballot submissions seems to be favorable for Brown as the raw number of absentee ballots in communities where he should do well have been higher than in areas where Coakley is expected to perform well.
These will be released as soon as polls close by 8:00 pm. Watch for the vote margin among Independents, the extent of the gender gap (Coakley needs to win women by a wide margin) what issues motivated voters (advertising, healthcare, abortion, etc). Also watch for the proportion of Independents who show up. If Independents outnumber Democrats, Brown benefits. if Democratic turnout exceeds Independents, score one for Coakley. Republicans (only 12% of voters) are expected to turn out in very high numbers.
One wildcard that the Obama visit may yield is higher than expected turnout among minority groups. Stay tuned...