So you want to know how to make sure you've covered enough literature searching to feel confident that you have found good stuff ?
Follow these three steps as a guideline to making your searches more comprehensive:
(1) Run a search in PubMed to find articles that you find helpful for your research topic
(2) Run a search in the Science Citation Index to see who else has cited the papers you found in PubMed
(3) Run a web search, ie, in Google Scholar to find additional resources that you did not find in the other databases
In step #1, you could also be searching other databases in place of or in addition to PubMed, such as: