Unless I'm missing your point, I would say the best way to find out is to go to the collections/card index search facility on the given website and type in the name of the parish or county and see what comes up, or more to the point, what doesn't. I don't think any of the subscription sites are going to tell you what they don't hold.
Personally when faced with the situation of not finding a parish register transcription on, say, FindMyPast, I go straight to
FreeReg and check with their records as these tell you exactly how much of each parish's registers has so far been transcribed under the Baptisms, Marriages and Burials headings.
For example here's link to the extent of the transcriptions for the parish of St Mary and All Saints, Beaconsfield in Buckinghamshire:
St Mary'sAlternatively, go to FamilySearch's Wiki and type in the name of the parish and see what it lists, eg
here for the same village of Beaconsfield.