According to his page on the Bleach wiki:
Tōsen boasts a great amount of spiritual energy. His skill in the use of his spiritual power is evident, as he uses his spiritual sense to "see".
He uses his reiatsu (spiritual power) to sense things around him, for example, to know where his opponents are during a battle. He should also be able to sense what the others are doing in the same way. It would not be otherwise possible for him to be a Captain-class shinigami.
The heightened sense of what people inside his bankai space are doing could also be one of the powers he gets from the bankai, because he has complete command over that space.