@LuisOlmeda, this a good thread right here. The "Answers from Developers" is not a live Q&A but bust a record of answers provided by Mike Curry heaping praises upin his favorite child. We are not that favorite child, but we manage okay. ;-)

Again, not an official answer, but here's my take on it.

I'd say the Dievas is invisible to all but their bonded Losejas, though certain knowledgable elders may know ways to detect the presence of a Dievas, as well as the Glamour knack lets the mage know that sorcery is present.  Other Losejas may know how to identify certain signs that another person has made a deal, mostly by looking for certain tells that are otherwise unremarkable unless you know precisely what you're looking for.

For the Losejas, the Dievas is likely a shadowy companion, existing just on the edge of their peripheral vision unless the Losejas wants to look directly at them, such as when asking for a Major Bargain.  The Dievas probably is whispering things to their host, not unlike Head!Six from the Battlestar Galactica update, but probably saving their comments for times when they might be able to tempt the Losejas into bargaining for greater power, similar to how the Imprint of Lascial kept trying to tempt Harry Dresden of the Dresden Files to take up the blackened denarius for several books.

I see them as a variant on the djinn. They are real, they exist and they will be seen when they choose to be seen. They will also not be seen when they choose to be. I also feel that they are not physically bound to the company of the losejas. The deivas could be thousands of miles away doing whatever they do. (They might even be extra planar) But when you call, they arrive and grant the boon because it allows them to effect the world. You may need to bargain, but the deal gets made so they can mess with things. And once the deal is done, they are gone until called. (Unless it suits them to make life weirder by sticking around).

I generally have them as invisible as long as they want to be. They're also not bound to the hip to their Losejas, they just have to respond to his or her demands when needed. Of course, they generally have a very limited ability to affect the world unless through a Losejas.

 

That said, it still leaves them a little wiggle room to screw with their Losejas. Like one of the examples given for a minor favor cost is a handshake with the Dievas. I'd generally give the Dievas some power to actually be visible during that handshake if they so desire, and even possibly alow them to appear in a different form during that handshake. There's a surprising amount of damage a Dievas can do by making sure their Losejas is seen shaking hands with some devil (or someone wholly inconvenient to be associated with)

 

One final note is that while Dievas are generally invisible as long as they want to be, Sorte Stregas could have some limited ability to detect them in relationship to their Losejas. For a Strega, the Losejas's relationship with their Dievas would show up in the strands just like any other, but there would be no discernable end point on the opposite end from the Losejas. It's not a surefire way, but it's a clue for any enterprising Strega trying to figure out who's a soreror or not.

From what I understand from reading some of Michael Curry's earlier posts on reddit, Dievas and their deals are going to get more fleshed out when the Sarmatian Commonwealth gets covered in the Nations of Theah 2 sourcebook.  Apparently Michael had one notion of how it'd work out, but somebody else is writing that chapter of NoT2 with their own ideas.