How do you guys assess a website? What kind of tests or issues do you look for while reviewing a website? The only reason I am asking is because I was asked by a friend to take a look at his website to see if there is anything that could be improved on and if there are any tips I can give. The reason he asked me is because I have a background in content creation for websites and know some bit of web development, but I feel very overwhelmed by this request and feel like I don't have a way of presenting my suggestions.

The only trouble is I want to give him a professional answer with some suggestions and points to help his website. But don't know how to do this. Any advice?