Most college professors have ways of determining if a work is from an "Essay Mill". I had a fellow student get bounced because he submitted a paper that he had bought.
When I was giving classes in the military I had a pretty good idea who was using "probability enhancement" to try to pass the class.
This is very true. To my understanding, many college professors even have a program they can run your essay through now to see if it pops up on the web pre-written. It's a very dangerous idea to play with, and one should never do so; it's not worth getting kicked out of school for.
Even in high school, most teachers can tell when a paper isn't from a student purely based off their academic performance. It doesn't take a genius to figure out that when a kid is aceing essays and bombing everything else, something else is at work.
Some of these aids online can be helpful, though, as long as they aren't writing the content for you. I had to read a monotonous civil war novel for my college US History class last semester, and I read about three chapters before just reading the Spark Notes (Online cliff-notes site if you're not familiar) and getting an 'A' on the test. In all honesty, I probably knew more about the book from those than half the students who either read it and didn't absorb much, or didn't read it at all. In times like that, these aids can be really beneficial. Whenever I read Shakespeare, whether it's for a class or not, (I love Shakespeare) I always use Sparknote's No Fear Shakespeare section which essentially summarizes and translates the text.
The reason I mention my practice of Sparknotes is because it seems to be the better part of the gray area of the using the internet to get quick and easy answers. However, I think Sparknotes is far different than turning in an essay you didn't write.
But, I don't see the harm in reading essays of topics you also need to write on, as long as you aren't pulling directly from them or stealing their ideas and words. I'm not a good analytical writer, so I struggle with college history essays, so awhile back I read a couple essays a buddy of mine had written for the same class. I didn't steal his ideas in any way, but reading an essay that was better than mine allowed me to figure out how to restructure my paper.