I've seen first hand people high up in startups monitoring what their employees say in assumed-to-be-private chats to each other. Technically it was part of the employee agreement that nobody read.
It's not a perfect, but one way to tell that the startup you're in is monitoring you is that they hand you a computer with your login already created. If you're opening up the Apple box yourself then it's much more likely to be free from keyloggers, etc. But don't trust shared sign-in stuff like Google Apps for Business to be private.
I suspect that the more likely you are to moan about the management, the more likely (1) the management wants to look at you because you’re always negative, and (2) the more likely the management wants to look at you because they’re the sort of control freaks that you want to moan about in the first place.
It's not a perfect, but one way to tell that the startup you're in is monitoring you is that they hand you a computer with your login already created. If you're opening up the Apple box yourself then it's much more likely to be free from keyloggers, etc. But don't trust shared sign-in stuff like Google Apps for Business to be private.