Dear Anil,
As a principal employer, you are ultimately responsible for your contractors' labor compliance. Hence, all you need to do is ensure that the contractor is providing the requisite benefits, maintaining registers, and following compliances. You can ask the contractor to submit their registers for checking every month before clearing their bill, and you should keep a copy as well.
Now, regarding leaves - if those workers have been working under the contractor for more than 240 days, they need to receive leave at a rate of 1 day for every 20 days worked; regardless of whether they have worked on your project for less than 240 days. Therefore, you just need to ensure that their leave register copy is given to you.
If the workers have worked for less than 240 days, then they are ineligible; unless it is due to discharge, dismissal, retrenchment, or if a worker passes away in service. In those cases, the rules are different.
Please let us know if you require further information.
Regards,
Deena Jagasia