Hopefully you have good documentation... err... some documentation... umm any documentation....
Documentation is so critical to understanding what data to expect, on what pages / actions to expect it, as well as providing context into why something is being tracked the way it is... sadly, documentation is something a lot of companies are missing, and that will make your life so much harder.
Knowing what tools you can use to test is only half the battle...understanding the expected outcomes is the larger and harder part of this....
If you don't have documentation (I've had to take over a site that had none, so I've been there), I like to create a document as I a testing... what am I seeing on each page, what variations are happening when I am logged in or out, what permissions are on the account (i.e. subscription based sites, or different user types, etc).
I look at the name of the dimensions / metrics... does the behaviour match the name / description.
I check the expiries of the eVars, do those make sense... and is Launch set up in a way that makes sense in conjunction with those expiries (for example, something like campaign codes, those have a 7 day expiry, once they are set on the page view, they don't need to be re-set on the actions on the pages that have campaign parameters... this would inflate the instance of the campaigns, etc).
There will be a lot of exploratory investigation, you will end up going down multiple rabbit holes, and it will take time to do a proper and thorough job...
Good Luck
