\previewOn has been available for many years, when activated, eforms typesets a rectangular box around a form fields so they become visible to one using a DVI- or PDF-previewer; recently, the eforms package (dated 2019/05/24 or later) supports an enhanced preview. When
\pmpvOn is expanded, any push button captions are typeset within the preview rectangle, and other form fields, the initial value, as designated by the \V key, are also typeset within the bounding preview rectangle.
Continue reading: enhanced-preview.pdf (Revised 2019/09/28)
The content of the above PDF is how it is seen in sumatraPDF, you will see only the outlines of the fields (not the fields themselves) as well as their captions and initial values. The source file is attached to the PDF; however, you will need the 2019/05/24 version of eforms to successfully compile it.