Every analytics implementation should have SDD, the SDD contains KPI's, report suite's, eVars, Traffic variables, events information, first check with your organization on availability of this document?
if you have SDD, then your life will be easier, you can find all your analytics implementation details out there. if there is no SDD then at least look for technical design documents. people who injected dataLayer objects at the application must create this technical design documentation.
If there is no technical design documentation, then the next source of truth to find out variables, events and traffic variables related information is your report suite, you need to go to the report suite settings and check for conversion variables, traffic variables, and events. if there is no excel sheet, then it is good to document all the information.
Do some regression on the website and identify what are all the page load variables are firing, you can use chrome debugger to check variable information. if any variable is not firing which you feel is not required then you can mark it for cleanup.
You can go to every event and check variable mappings with JS objects. you can check this either in DTM or Launch, it depends on what your organization is using the DTM is going to be deprecated, it is good to use Launch or migrate to Launch.
Open custom code editor and check for custom code, plugins, etc.
You also need to check classifications, preprocessing rules, anything at the marketing channel level.
Check any reports that are already built at the workspace level to get some idea.
If they already using Google Analytics, web trends, etc, then you need to understand that implementation and find out all use cases and KPIs based on that you need to start your implementation. it is good to start the new implementation with SDD.