I already managed to create function which checks if user is using a trial/demo or full/purchased app on Windows Phone 7.
Is it possible somehow to know if user has reviewed or rated a current app?
I don't believe this is currently possible, you may be interested in this blog post regarding reminding the user to leave a review for your application :-
http://roybott.com/blog/getting-ratings-for-your-wp7-app.html
You could try the following :-
I think the only way to do that is to save a value in the isolated storage, such as: hasRated, and its value is initially false.
Then when the user presses on the menu item: rate me! that value will change to true.
Before deciding to show a reminding message or not, that value is checked.
Unfortunately, you will not be able detect if the user pressed "rate me!" then pressed back button without rating, he will be considered then as if he has rated.