How can we verify the validity of a plagiarism checker?

Before being accused of copying work from the internet recently, I am a student who has never been frightened to write. I didn't copy any text; all I did was include certain references, which were flagged as plagiarised. That occurrence made me feel so humiliated that I'm now interested in learning how to utilise the online law essay writing service Checker For Students UK-based so that going forward, I'll know that my work is original and how to verify the legitimacy of a plagiarism checker.