This is not really a css question as such and out of my comfort zone but I know that clients expectations are always hard to manage
I would have thought that by now (the end of the project) that you would have established enough of a relationship with your developer to realise if he was doing a good enough job or not (unless they are good at pulling the wool over your eyes). You usually get a feel for whether someone knows what they are doing or not (this applies to clients as well as developers).:)
As far as validating goes then you can easily run the code through the w3c validators:
However unless you have some knowledge of coding you will not be able to interpret the validators findings very easily. It is rare for a site these days to pass validation because of Vendor Extensions and other proprietary code that goes into the production of a site. While code may be invalid it doesn't always mean that it is coded incorrectly and on the other hand code that is valid doesn't mean that it is good code.
The validator will show obvious mistakes such as missing tags and broken html so these are the things you should be looking at but it still won't tell you if the site is coded in the best way. You can make something with perfectly valid code but you may be using incorrect methods and unless you have some knowledge in that area then it will be difficult for you to decide if it's fit for purpose or not.
As far as performance is concerned there are some performance tools you can download which will help with working out any bottlenecks and best practices.
You should then discuss the findings with your developer to see if there is anything you can improve but it may be that the requirements of the design have forced certain decisions along the way which may impact performance (e.g. clients always want higher quality and larger images and fancy fonts).
It also depends on what design features you have asked for and you would need to check the site meets those requirements. e.g. Is it responsive, accessible, usable ? You can check those things yourself by just using the site and stress testing it and see how it behaves.