Wall of text incoming, and I've two subjects to bring up.
The "too long; didn't read" version is the following:
1a) I believe that knowledge of the very existence Shadow Weave in our server time is likely to be limited to very few people, who would not be spreading this knowledge for various reasons.
1b) I'm not sure how noticeable the differences between a Weave and Shadow Weave caster is, but it seems that it shouldn't be visibly apparent.
2) There's some inconsistencies in how reliant divine casters actually are on the Weave, namely whether they even need it to function. If anyone could clarify, now's the time to step forward.
See, now you can make input about the matters you care about!
First Matter
Something to keep in mind is that the very existence of the Shadow Weave might yet not be widely known. The Magic of Faerûn sourcebook has the following to say on the subject:
"While the Shadow Weave has existed for quite some time, only recently has its use begun to spread rapidly and its power become known to many." (page 7 if you care to/can check).
This opens up the possibility of debate what constitutes "recently" and how many might actually know of it. I would argue that most people wouldn't know of it yet, and my reasoning is the following:
The people who find out about the Shadow Weave generally come in three categories:
1) Shar's own worshipers.
2) Those who are allowed to discover it as part of Shar's schemes.
3) Those who accidentally stumble upon it during their Weave-based research.
If you're part of 1, you're not going to talk about Shar's secrets to people no matter what.
If you're part of 2 or 3, spreading something that Shar wishes to keep secret (so far) is to invite terrible retribution from her devotees. Indeed, it is mentioned that most who do so keep it to themselves for various reasons, of which I suspect this fear is the primary one.
I'm thinking that the Time of Troubles seems like a logical break point where knowledge might start spreading; when Mystra briefly dies, Shar could be among the first to try to use the sudden power vacuum and start to offer the hordes of now magic-less casters an alternative that might work once the deities are restored. In the end a new Mystra is chosen, but the offer has become known and knowledge of the Shadow Weave could now begin spreading for real and evil-aligned casters start flocking to it.
Finally there's the matter of if Weave casters would even notice any differences in a Shadow Weave caster. I've managed to find nothing that explicitly says whether the visible effects change enough for normal casters to even realize something's wrong. On the other hand, there's no evidence that it's
not noticeable either. The closest thing we get to anything is a Shadow Weave-related feat that makes certain spells they cast harder to detect via divination magic; but that doesn't extend to or even state to begin with that the magic is visibly difference.
There's a lack of lore (that I've found) to support it going either way, which I don't quite like, and if anyone has anything saying one way or the other I'd be glad if it was shared here. The only difference I've read and can remember for sure is how it feels for the caster to cast Weave and Shadow Weave respectively, which obviously doesn't include visible clues.
I'd argue that it'd make some degree of sense that it's not as easily visible as just looking and listening, as it's essentially a secret and masking it as the ordinary would go far to keep it. Instead, it would require more in depth analysis. For instance, there's mention that when you cast with the Weave, you create some sort of rippling effect in it. Finding out a Shadow Weave caster by noticing that there's no such effect on the Weave, via detect magic and a Spellcraft check combined, would be a good way to go about it.
Second Matter
Second, here's something that might possibly require some pretty in-depth research to sort out. It probably goes more into the exploratory territory more than being a real question as well, but perhaps we can reach a conclusion here.
On one hand I've read an explicit mention about divine casters that goes thus: "Unlike masters of the Art, those who use the Power have
no skill in using the Weave. Their spellcasting knowledge is
planted directly in their minds by deities in response to their
fervent prayers" (Magic of Faerûn, p. 8). Note that it doesn't say that the Weave is entirely unnecessary for them; merely that they do not need to know themselves how to use it.
For the uninitiated who wish to make some input, "the Power" refers to divine magic and "the Art" to arcane (and both these terms are indeed those used by in day-to-day speech on Faerûn; good to know!).
Magic of Faerûn also has this to say (p. 11): "Mystra can also bar a deity from accessing the Weave, which would prevent the deity from using magic while in Faerûn but not while the deity was on another plane where the Weave does not exist, such as any of the planar homes of the deities. Such an act is very draining to her, and she only does it to reduce the power of a deity intervening directly in Faerûn.
She cannot block a deity’s ability to grant spells to worshipers without negating the ability of each worshiper to draw on the Weave."
As I see it, there's some problems caused here, namely as to the degree divine casters are themselves reliant on the Weave. The implications I've read seem to say that the Weave is indeed necessary for all magic, arcane or divine, but it's partially circumvented as the deity simply provides everything necessary to the divine caster - all the caster needs do is trigger it and the Weave conforms accordingly. The second quote above seems to me to imply that if a divine caster was denied Weave access, the attempt to trigger the magic they have been granted would fail just as if he were an arcane caster, which would make them as reliant as anyone else on it functioning.
Again, if anyone knows the actuality of the matter, I'd be really happy if it was shared here as it's likely an important part of magic philosophy on Faerûn.