Well that's the problem - if only it were as simple as knowing that, if it's a National Rail service you'll get the discount.
There are plenty of legitimate National Rail journeys which are priced on the Tube fare scale. How are you then to know what discount will be applied? And when do interavailable routes mean you get a discount? It's a minefield.
It is as simple as National Rail service means you get a discount - the exact fare can be worked out using the fares calculators, this will give you the overall public rate for journey. The Priv NR Only discount will then be 75% of that public fare, it doesn't matter which fare scale your journey uses, as long as your journey is entirely on National Rail and doesn't use the the tube or DLR you will get 75% off whatever the public fare is for that route.
The limited cases where NR Priv Oyster is applied to LU/DLR journeys can then be found on the last page of this document:
https://www.raildeliverygroup.com/files/Publications/services/rst/RST_NR_Only_Priv_Oyster.pdf (an out of date document, but that list is correct). All of these are journeys where the tube/DLR route parallels a National Rail route, but it clarifies a few situations where LU services use a neighbouring station/different gateline.
I agree it would be good to have a proper calculator (even an unofficial one or just in the TfL Open Data for third parties to provide), but in terms of knowing when the discount will be applied this is as simple as NR = 75% discount, journey involves LU/DLR = no discount - there's a few edge cases on LU as detailed above, but they are limited enough that you could just take them as an unexpected bonus (or if in doubt just use the parallel NR service where you'll definitely get the discount...)