Rasdan, we need to distinguish between old Madura and recent Madura.
With Filam's keris, we are looking at recent Madura. I've seen a great many blades of this type and quality and for me, its more or less a matter of --- click! it fits the template in my mind. end of story. But even then, I have been wrong when doing this from a pic. I have no doubt that in the cases where I have been wrong, I would not have erred if the blade had been in my hand, but it wasn't, I was looking at internet photos.
However, for old Madura it is no different to deciding on a tangguh for Mataram or anything else. This page in my site gives a reasonable explanation of the indicators that we consider.
http://www.kerisattosanaji.com/keristangguh.html
Probably about all that we can use from a photo is :- pawakan, gandik, point.
The point is long, ie, distance from last luk to point, the gandhik is steeply angled, the pawakan is stiff and too upright , straight awkward gonjo, rotan cross section.
Madura is one of the easier ones to do from a photo, because the gandhik is very steeply angled --- but sometimes its not, and then it becomes impossible to do a tangguh from a photo, you need to hold the blade.
I've said it time and time and time again:- you cannot do tangguh nor learn tangguh from photos and written explanations. Its a guessing game, and the only way to learn the rules of the game is to spend a lot of time with somebody who understands it perfectly and who has access to a lot of good quality keris.
There is no other way.