police verification procedure

Respected sir / madam, I am from Westbengal wanted to know about the detail rules of police verification of a govt employee means either candidate should have to do this from their end or it's a govt process...? Again is it too important....? if not done in someone's whole service life ? My wife is a staff nurse and rendering her duty since from last 3 years but police verification not done yet. So is there any chance of problems?? And if police verification is must have to done from the candidate's side than eager to know what's the details procedure of police verification step by step ? Thank you.