As I'm not a native speaker, I often see that my posts (either questions or answers) get edited. Sometimes people edit my post to fix spelling mistakes / language use errors. What do you think about an option to be able to mark the typos / mistakes that people make in their posts i.e. with a red font? Or, maybe provide a post version history so people could see what the errors in their posts originally were? There is of course the spelling dictionary, but the dictionary fails to catch all of the errors (like phase context related questions or tense related errors). On the other hand, if this feature was already present, I don't know if users would not abuse it to i.e. mark everything in red. What do you think? Also, there is a concern if Stack Exchange should be a content providing site or a site that teaches English language to others.
If you see a post that has been edited, if you click on the "Edited [Date]" link you will see a list of post revisions. I'll use your question here as an example.
If you click on the link as shown in the red box:
You can see (in red) what was taken out and (in green) what was added.
For more info, have a look at this FaQ entry, "How Does Editing Work?"
Since others have told you about the edit history, I will speak about editing in general.
I wouldn't worry too much about this, (most) people don't edit posts to stick a finger at you but to make the site more readable for everyone. This is important because StackExchange is not meant as a short-lived forum, but we try to create a body of information that has lasting value. Any text is more usable if there are fewer spelling/grammar problems as these can drastically reduce the readability due to the wiring of our brain, which throws an exception every time it notices an error and that has to be handled, thus reducing execution (reading) speed :)
I am no native speaker either, and it took some time getting used to the fact that anyone could edit my posts, but I eventually just came over it and accepted that this was for my benefit as well as others.
(BTW: Salutations and closings like "Regards" make it all more likely that someone is going to edit your post, as these are not wanted here because they don't add value.)
If you click on the date of an edit, it will take you to a page with the whole revision-history of the question:
Leads to this:
I think this already exists. If your posts has been edited then you will see a notification to that effect.
If you click on the date of the edit then you will be taken to the /revisions page for the post in question in this case https://serverfault.com/posts/347886/revisions which shows additions in green and removals in red.