If you know you are doing a good job, and you have taken away any doubts (click here if you are still not that sure) it's time for you to test your grammar and vocabulary (links on the pictures below).
These two tests give you only an idea of what you are supposed to know about grammar and vocabulary, but they are not enough. Remember that in a certification exam it is always very important to practice on the template used in that specific test: knowing before the way you will be tested represents a time saver during the exam (the scheduled time is not much) and helps you to keep relaxed (anxiety has never been helpful).
Take for example the sentence transformation (Paper 1, Writing exercise 1): it can become a tricky exercise, if you are not used to it.
You can download a sample clicking on the following link Download sample papers for Cambridge English: Preliminary (B1).
If you want to continue practising, there are more examples of PET exercises on the test page of this blog.