You have identified one of the issues with the current version of ANSI/ESD S20.20, specifically, what is an isolated conductor. The ESD Association is working on a revision to ANSI/ESD S20.20 that hopefully will help with what an isolated conductor is. Since it is still in revision, I cannot give you a definition as it might change before publication.
On your question, if you define an isolated conductor as a conductor that contacts an ESDS item that measures greater than 1x109 ohms to ground then you need two conditions. First, you must identify the conductors that come into contact with the ESDS item. Second, they have to measure greater than 1x109 ohms to ground. If you claim that you have identified all the conductors and all of them measure less than 1x10*9 ohms to ground then you do not have any isolated conductors and do not need to make any further measurements.
You do have the equipment to measure the resistance to ground (I assume since you stated this) so you do have measurement equipment to identify isolated conductors.
If you do ever find an isolated conductor then you will need to make the measurement with additional equipment.
I hope this helps.