How do I test grounding/lightning protection?
Well, there are special measurement devices... I just did a quick search for something available in the US
https://www.fluke.com/en-gb/product/electrical-testing/installation-testers/fluke-1664-fc
With such a device, a trained electrician can test a local installation to provide values which are covered by the standards, rules and regulations.
appart from that you may want to look into lighting protection, here I can name you a company I know and which I use in my house
https://www.dehn-usa.com/en-us
What you want to have is a cascade of Type 1,2 &amp; 3 Surge protectors for the Power supply (with Type 3 is in your UPV, or in multi sockets).
also you can do more for IT Equipment
https://www.dehn-usa.com/en-us/surge-protection-for-it-systems
like connect screens of the Ethernet cables with ground more often, so you get a mesh with low imperdance.
https://www.dehn-usa.com/en-us/surge-protection-for-power-supply-systems

Especially make sure the customer connects all (!ALL!) earthing points e.g. of switches with GND using SHORT single core wires of appropriate diameter to the ground rail (good cabinets have more than one).

No need to mention: measurements and installation should be carried out by trained and certified persons!