You can check using softwares such as Turnitin, Urkund etc. The toughest criteria used by this software is matching any three words in a sentence at the same place. You can adjust the settings to view similarity index under different conditions. Commonly, similarity index should be below 30%. If you write the thesis by your own after understanding the literature on your work, you need not to worry about plagiarism. But if you copy sentences or paragraphs from literature and insert into thesis as such or editing here and there, the software will definitely expose your plagiarism.