Pretty much the same way I witness to Catholics who claim that only Catholic bibles are correctly translated. I quote their bibles to them. Someone later in this thread says it is more about interpretation than translation. I have made that point to Catholics recently when I was challenged on John 1:1, see my link on McKenzie in my signature for that post.
As for KJV-only advocates, it really depends on why they trust the KJV. Many are greatly surprised that I have a copy of the 1611 edition that was also mentioned later in this thread. It contains the Apocrypha and many are stunned to find out about this. So when they say they use the KJV I do ask which edition which confuses them. I tell them I use the original, not a later copy that was changed. I ask them who changed it and who had the right to change it. Also, the translators preface is very good. In it are discussions about why certain readings or renderings were used which shows the process was not cut and dry. They also clearly don’t claim inspiration.