Find the Greek text a version is based on

NB.Mick said:

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.

Adding to Mark, the easy way is use a linked text comparison to highlight where the translations diverge. Then a basic apparatus that shows the key differences in greek (eg the SBL greek apparatus).  A more technical apparatus (eg NA28, USB5, etc) generally sequences the greek differences in traditional MSS order, not used order.

Of course, Metzger's comments are good pointers where arguable.