The Sox are currently 28-12, for a .700 winning percentage, and are 2 games better than any other MLB team. To win 100 games all they need to do is go 72-50 (.590) the rest of the way. Over the course of a 162 game season, .590 ball would bring 95 wins, which they've done 3 out of the last 4 years.
Incidentally, the Sox are on pace for 113 wins. They've won 100+ games 3 different times:
1946 (104 wins), when they went to the World Series.
1915 (101 wins), when they won the World Series.
1912 (105 wins), when they won the World Series.
They've lost 100+ seven times.