The easiest way to measure the performance of your project manager is through the achievement of project milestones.

Generally when people hire a project manager, it is so there is one person responsible for all aspects of the project to bring it in within scope, on time and on budget. They should have enough experience and expertise to create a project pan that is achievable and balances scope, cost and time. The project manager should have a clear schedule, the "critical path" identified with the key milestones to be achieved that indicate that the project is on track.

If you have reviewed the plan with the project manager and agree that their plan looks achievable, then the attainment of those key milestones should be the primary way to determine if the project is on track. You can also look at the scope that has been completed to date based on the plan and the money spent based on plan.

Feel free to give me a call if you need clarification. I can also provide tools and templates if that would be helpful for you.