so, pacts with hungry cosmic outsiders, domineering kraken, and soul-repurposing devils are taboo, right? that's what makes those cloak and candle parties in the noble houses of waterdeep so invigorating: the power and the danger and the... taboo. the forbidden-ness. forebodence? anyway, what's the cost of being found out? there are plenty of campaigns with baddies like tiamat being summoned up and cultists (with greater or lesser commitment to the cause) behind that. aside from a few concerned public figures hiring out adventurers to investigate, who exactly is waggling fingers at this to make it so forbidden? what societies/groups/clubs are actively hunting down cultists in ferun and other settings? or your own setting?

and all that to ask: what's that mean for a warlock in your campaign? are they hunted? are they determined? are they as cautious as a changeling, careful not to reveal themselves to outsiders? are they simply another career choice like plumber, potter, or traveling mace-swingin' adventure priest?

This is a very good question. I know that often there are law enforcers but introducing secret societies make it interesting. In a faerun campaign I’ve run I had a secret branch of the lords enclave that were inquisitive and mastermind rogues. One of them found out a character was a zhentarium member and after trying to kill that pc, he tried to bribe the character to use that to gather info for the lords enclave

other ones from cinema include the TVA from Loki that make sure the timeline goes how it’s supposed to. 

I’d try any of these options if a warlock or evil character is discovered or exposed.

Hostility in community: maybe not sell to PC or in rare cases try to imprison or murder

Taking advantage of it: NPC might use the PC as a shield or throw blame on.

Religious involvement (my favorite): Priests or churches step in and want to save the characters soul. This might take character through religious rites and atonements, forcing characters to write apologies, the possibilities are endless. It has some hilarious role playing and the stereotypical priest looking to convert everyone is just entertaining.

seems like there should be something worth gleaning from official adventures. for instance, Waterdeep Dragon Heist and a copy of the city laws. seems like unless just joining a cult could be considered blasphemy, then the City Watch would have to have evidence of assault, murder, or disturbed peace. the Order of the Gauntlet faction has a thing about hunting down evil, but probably not in the streets on suspicion alone. Emerald Enclave would probably attract Horizon Walker and Monster Hunter rangers with an interest in anything leading to or resulting from a rift, intrusion, or summons... but again probably not just people with the wrong neck tattoos in the streets.

the same adventure also includes a bit about a Waterdavian cult of Asmodeus which seems to do more to put tieflings in a bad light than clarify how much trouble an unmasked cultist would get into. that cult seems to be separate from the Ashmadai, the Neverwinter division of Asmodeus worship. according to the forgotten realms wiki, they seem more like a gang and more likely to be picked up for crime crimes than just their ideology.

with nothing better to go on, seems like inside Waterdeep at least a revealed dragon/fiend/aboleth patron warlock might elicit whispers and even a tail. outside the gates (or even behind closed doors), i wonder how far courtesy extends.