What is the effective technique to evaluate pmo performance in your opinion?

Successful PMO means that projects are delivered:

1. on time

2. on cost

3. on scope

4. on quality

5. on beneffits

For the first4 items you can design a post-implementation survey to the projects' key stakeholders in order to evaluate performance in these4 drivers. For the5th and last driver the benefits review plan is probably your best source of information

thanks , but Nawzad did you actually applied OPM3 ? or suggestion?

OPM3