What is the best/easiest way to find the Greek text a version is based on?
Read the foreword.
Which isn't usually included in Logos editions…
Back to the original question: there is no one 'Greek text'. There are various families of texts, which we tend to group into two familes: (1) Textus Receptus/Byzantine, and (2) Eclectic (Nestle-Aland/UBS). There are several (slightly different) editions available of both 'families'.
In general, the KJV and its children (e.g. NKJV) are based on Textus Receptus, whereas pretty much everything else is based on the eclectic text. In practice, the translators will be making decisions at various points that differ from the underlying Greek text, so no one English Bible will follow 100% any one Greek edition.
Thanx much.
Bear in mind that Faithlife don't necessarily use the same Greek text for their Reverse Interlinear. You can usually get that from Resource Information, under Reverse Interlinear.