[e.k.]

Tx for that hint.
If you use this option, you loose permanentliy a big part of the view port for scores.
Displaying text may not be so diffucult, but with score sheet, I dont want to decrease the size of the "paper".

Maybe this will work for you (in my case: I just want to know, is there one page or more pages)

Add to 
"Library Setting/Song Title Formating": %PAGE_COUNT% (title format or caption format/show caption)
(You will see number of pages in all lists. Advantage: You see number of pages before(!) you open a pdf)

And/or:
Set "Display Settings/Page Slider Visibility": ON
If there is just 1 page the page slider will not be shown.
If there are more pages you´ll see the slider near bottom of screen.

I´m not sure, if you asked for such a kind of "indicator",....