Searching for best/common practices, in my opinion, is always the best starting point, well done!
As a second step, I always ask to clarify what are the UX KPI's that the client wants to check.
You can do that directly with the client and/or with the project owner, business analyst etc
With the KPI's validation it will be easier to define a specific list of items to check, for instance:
1. responsiveness for all the covered breakpoints
2. consistency across all the pages, modals and views
3. consistency of the tone of communication