ISO 20000 contains many required records ,documents or other information. So, it's hard to generalize. In e.g. Incident Management or Change Management processes - you will look for tickets related to incidents i.e. changes. Same for Problem Management. Availability or Capacity Management will require measurements - so you'll look for measurement files.
Further on, Configuration Management requires CMDB. So, you need to check whether it exists, in which form, does it fulfill standard's requirements...etc. CIs need to have recorded incidents and changes...so, these are all items you need to check.