Project Server PSI - Insert or update

Hi

I have observed that project server behaves differently at the time of insert and update. during PSI call I was using the QueueUpdateProject Method to update the certain custom Field, But it does not affect untill there is already a row then and them update method is effective.

My Question is how to identify that Custom field A needs to insert and Custom Field B needs to update. and set the project dataset in order to take effect the new changes made into the project.

any suggestion???

Thanks, Parth