Reliability of HSK practice tests
I have done HSK3 and am preparing for HSK4. I used some HSK3 practice tests to see whether I was ready for that test. When the practice tests are identified as past test papers, they should be a good rough guide to your readiness for the real test. I treat them only as a rough indicator because there are no exact descriptions of what you need to know for HSK tests (except for character lists, which are only marginally helpful). 

You can also find HSK preparation books. The practice tests in those books are sometimes somewhat harder than the real HSK test.