I passed AWS CSAA examination this week. The Whizlabs practice tests have been very helpful. My score in these practice tests was actually how much I scored in the real exam !
However, the practice tests questions are a bit simpler than the real ones. This is a general observation - they do contain several tough questions.
I suggest to look at Section Tests - these are good indicator of the kind of questions asked in real ones. Many of these questions helped me fill the gaps.