magpie
There is little to no difference in head and jaw shape between Senegalus senegalus, Polli, Palmas palmas and Palmas buettikoferi as they're all members of the Senegalus group which share similar headhsapes (with the exceptions of Delhezi (not mass bred) which have flatter heads part of being a transitional upper to lower jaw species, and Senegalus meridionales which also has a flat head)
Retropinnis on the other hand do have very different headshapes to the others, being a member of the Weeksii group, they also have wider heads and mouths.
Another nuance which you can sometimes apply to the Weeksii group (with the exception of Teugelsi and Mokelembembe) have a more squared facial structure if you understand. The ridge from the eye to the barbules or mouth is quite angled, same as the neck and cheeks. (although the more captive bred it is, the rounder it will be)
The Senegalus group, as you can see in this photo of the Polli has a very round cranial structure.
You see where it begins to flatten on the forehead, well use the gap between the barbule and eye to identify. On the Senegalus group, that gap transitions smoothly from the round cheek to the slightly flatter head. Often on the Weeksii group, that gap doesn't transition as smoothly, so you see a harsher angular shape there. Hopefully that makes sense.
Besides broadness of head, there is no definitive way to separate the Weeksii group and Senegalus group on headshape alone. There are tiny little factors you will pick up on though such as the ones stated above, but the longer you keep them, the more you will notice.
My process if applicable is:
Check jaw protrusion,
If it's upper, check head width,
If head width isn't obvious, check for those secondary nuances in facial structure,
Then I know what group it is and study body markings, facial and cheek features.
If the markings aren't always obvious, count the finlets
If after this, you still struggle to see it's part of the Senegalus group, it might just be a case of keeping the groups longer
