hiya memo, ive shopped someone before and do i feel guitly....heck no although i was very disheartened to find that noothing came of it and they appeared to get away with it. It is confidential, you can iether do it online or by phone. All they need is a name and where theyre working, any extra info is welcomed such as times that they start/ finish etc.
I wouldnt worry too much about being narrowed down, afterall if they are brazen enough to do this then they should be aware of the consequences. There are many decent people who cant stand for people doing this while everyone is meant to struggle on with life by there own means, therefore someone will shop them in time even if you dont.