The below largely covers what's been said already:
The neighbors screen is the first step, after ensuring visually the enemies are present. I don't really use the neighbor screen for enemy range purposes, though, but for non enemy trade access (if listed, they ought to send caravans), as well as to detect if the goblins are at war with me or just generally hostile (I haven't seen any difference between war and general hostility in the sieges, though; they seem to be about the same). Obviously, you can also see if you happen to be at war with elves and/or humans, which happens occasionally.
The best way to attract goblins is to embark closer to them than they are to any other settlement. The same should go with black towers.
Black towers have no pop cap threshold: they can arrive in the first summer. Also, undead armies are badly bugged path finding wise when arriving, so there is a large risk the army will drift randomly along the embark edge for a season or a year before leaving, without ever getting near your fortress. Undead that happen to stumble on your fortress rush in, but the do not communicate, so the rest won't come, unless one that stumbled on the fortress entrance happens to be one of the living handlers, in which case the handled undead follow the handler.
Pop cap thresholds are easily changed (I wouldn't even call it modding, since it's more tuning). Check the wiki for PROGRESS_TRIGGER_. There's one set for each civilization type. There is a similar parameter for titan/megabeast attacks that also defaults to 80.