I suggest you as a Principal Employer (PE) devise a checklist what compliance/s (PF/ESI/Insurance under employee compensation Act, minimum wages act, payment of wages act, payment of bonus act,and so on) are expected of you under different statutes. These should be positively checked before releasing the payment to all contractors working within your premises. Once you are satisfied that all such compliance/s are met with by the contractor/s then only monthly bill should be released. This should be a continuing exercise, every month. (I have even seen contractors keeping two sets of pay-sheets for reducing the burden of bonus; but that is another matter) The point is contractors are generally lax in payment of various dues or filing the returns due and expecetd of them, liability of which can devolve on the PE. I have advised this before very recently on the same subject, in this forum.