What is the best way to find plagiarism in a draft research?
It depends what you suspect was plagiarised, how and where from. 

If you believe the ideas/structure/references (the latter would apply to a literature review) have been plagiarised, you're only recourse is to use your expertise and knowledge of the field to identify the sources. A literature survey of your own would help this.

If, as I suspect, you mean the text has been plagiarised, you can use services such as Turnitin and iThenticate (as suggested by others). If you work in a university, it is very likely that they already subscribe to one of those services, and you should be able to access it. Very often, universities provide (password-protected) access for students to check their work through such services, prior to submission; usually through their virtual learning environment (VLE, e.g. Moodle, Blackboard). Your librarians would be able to point you in the right direction.

However, there's a much easier way: just Google (and Google Book search aned Google Scholar search) the text you suspect of being copied. Pretty much the only sources Turnitin et al. have access to that Google doesn't is previously submitted student assignments; so (as I read this case) Google is as likely as anywhere to identify the source of any suspect passages.

I just picked a random sentence from a nearby textbook to test this. While "straight" Google didn't pick it up, Google Books did.

When I was a fresher, and we had our "don't plagiarise" lecture, I remember the lecturer telling us that the university subscribed to all these expensive anti-plagiarism services, but the lecturers just used Google instead.