Is it correct to tell a professor that solutions to exercise sheets are online?
I reckon most professors know that the solutions are available online. Nevertheless, telling them is a nice thing to do. It's a way of showing, that you care about the quality of the lecture, and it might even trigger a fruitful discussion. 

I am well aware that the solutions to my problem sheets are available online. Students told me so, but it is also obvious from the number of correct and wrong solutions. How do I know this?


I use a random number generator to obtain different starting values for the problem sheets. Students, who don't pay attention, might get it wrong.
Whenever I add a new problem this particular question gets the least correct answers. 


Now, filtering the scores from old problem sheets according to the two criteria above and correlating them with the scores of the final exam, it gets pretty obvious that "doing the homework" and "coming to "consultation hours" significantly influences the final grade. I show this data every semester, at the beginning of a new course. During this presentation I also mention that the solutions are online, however, I try to put this into the context it deserves: Trying to cheat on the problem sheets is a waste of time. The time is better spent, having a beer and discussing the subject with your fellow students.