I am self studying for the SAT so how should i know whether my essay is really worth a nice score?
I would definitely read [this resource](Understanding SAT Scores – SAT Suite | College Board) provided by the CollegeBoard. Other test-prep services can shower you with all kinds of advice, but you can’t go wrong with following the rubric of the test-provider.
Make sure you read the examples CollegeBoard provides (Example 1 and 2) for each score. They were an immense help for me personally to gauge what scorers were looking for in my essay.