How is the test graded? 

The Reading and Listening sections are graded automatically by the testing platform.
The speaking and writing sections are graded by a human grader, according to these criteria:

Writing: Overall Response, Linguistic Range and Accuracy, Coherence and Cohesion
Kaplan scores range between A1 to C2.

Speaking: Overall Response, Linguistic Range and Accuracy, Fluency

Information on Kaplan Test Scores can be found here.