IMO, the "Best" way would require an on-site audit as soon as possible after installation is complete...running down the checklist for install and the final approval checklist to verify that all steps were done and the result matches the intended result.
This would audit the installation process, and likely include some of somashekar's questions above.
That said, a functional test ...actually USING the equipment would perhaps be the most pertinent test as to whether the install met customer expectation. This is not an audit of the install process as much as a verification that customer expectations were met. It would include customer feedback as well...though that should really be part of the initial install in the first place..."Are you satisfied?".
Short of going on site with the customer, a debrief with the installers themselves at home base is another way to run through the process...albeit a layer or two removed from the actual target. Pictures from the install and a customer sign-off might be valuable additions to this.
Just thinking out loud...
