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It depends on how you work and your relationship with the client.  I guess the first step is identifying issues and then finding the right person to fix. If you look around you'll find plenty of (free) plugins that can help you if the platform is Wordpress. That list is not complete just some main points that usually do some good, and it is in no particular order. This is a must - https://moz.com/blog/technical-site-audit-for-2015