Alex,
For CPL you are not required to fly with 170A approved FI/FE. For CPL the FI/FE can actually sign the paper work just on the basis of training required. All they are signing is that you have completed the required course and have met all the requirements for the CPL skill test. However, it is highly unlikely that an FI/FE will sign 170A without flying with you.
For IR you must fly with 170A approved FI/FE before he/she can sign your paper work.
In both cases it is like a mock skill test and will last for about 2-3 hours. I am sure your CPL/IR instructor will go over everything and will explain to you what an examiner is expecting from you for 170A.
Good luck.