To be honest they probably can through a variety of methods, but most of them take a lot of tecnical knowledge to use in my opinion.
The question is, why do you want to know if they can monitor your content? Privacy worries? Doing something you shouldn't be? If you were just curious then yes they see what you are doing theoretically. If you are worried they are checking and want to defend yourself then I am sorry but I'm not sure of where to go from there. I'll do some thinking and see if I can come up with anything in the meantime!
@JogaBonito1502 You don't need to have anything installed on the target computer, there are a variety of methods that can be used, one of which essentially hijack the packets as they travel between the target computer and the wireless router and decrypts them. But it does take quite a lot of technical knowledge and the ability to code in C/C++
Oh and sorry if this is of absoloutley no use to you or I'm just being stupid, I'm only trying to help out thats all
I'm quite new to the whole offering support kinda thing :P I just hope I was of some use! Sorry again