How do journal/conference reviewers make sure of the results' integrity for submitted research papers? How do reviewers/referees make sure that the author has done the work as he described in his paper and the written results are achieved?

Do they ask for the code or the tools that you have used? Will they test the output to make sure that the results are not faked? 

I know that reviewers with 20+ years experience in the specific field can figure out if the results are real or fake or if they are even accomplishable, but it is still easy for the writer to fake results without any monitors.. 

For example in CS machine learning, you can extract an extra feature from the texts and claim that you got better results in classification than others have done before, when in reality you did not. So how do reviewers make sure that your work is sincere?
They don't. Reviewing papers is volunteer work, which has to be done besides the regular job. So no more than half a day is spent on reading the paper and writing the review. That is enough to filter out obvious scams. But for more sophisticated fraud we rely on people trying to use the results once published and finding out that it does not work. The threat of the subsequent sanctions (and for most, the internalized honor code) is hoped to prevent most fraud.