How do you know the project you are working on is worthy of a PhD dissertation. How does it differ from MS dissertation I am speaking of project scope here.

So suppose my advisor is working with an industry .And you are asked to build a prediction model for small section of that industry? Does that qualify as a Phd Project or a Master's thesis project.

Or does a Phd project has to be generic model , in the sense all companies in that niche can use that model as basis for their analytics?

I am trying to understand the differences in scope for a PhD and MS Thesis project.
I've had this discussion recently—the major differences are in the scope and independence of the work. 

There is no easy, absolute method to say "X is a master's thesis" and "Y is a PhD project" without knowing the amount of work involved in each approach, and how independent the student needs to be.

Ultimately, a master's thesis represents a body of effort designed to show that you have successfully completed the project to the satisfaction of your supervisor. A PhD thesis indicates that you have produced a sufficient intellectual contribution to your field, and that you have the capability to be an independent scientist.