How do the police know if you breach your bail conditions?

[Wukong]

the short answer is - don't get caught together. Don't break your bail condition. Such a bail condition is usually set to keep someone like you out of harm's way.Was the bail term "don't speak" or was it "don't associate".
How you most likely get caught is - someone sees you, like a cop, off duty cop, or "concerned citizen".
Someone with a little spare time may check phone or internet records - it only takes a couple of minutes, and there is nothing you can do to hide it.

Unless your boyfriend is really big time, or part of a bigger organisation, there won;t be teams of Agents with camera's on every roof top, sophisticated electronic surveillance, and Steven Segal is unlikely to be under your bed (but have a look to be sure !)