Lanefan
Victoria Rules
For me, I see it as being on the players more than me-as-DM to move things forward, as they're the ones with the supposed protagonists under their control, and if they can't find any clues here they should try looking over there...or just start breaking heads until one of those heads has something useful to say...or abandon the investigation and go do something else.As far as when to nudge the PCs along, if they are enjoying themselves, sometimes it's fine to have a slower, less action-packed adventure. Sometimes it's nice to just enjoy exploring the world and role-playing your character. However, if the PCs are starting to get frustrated or bored, then that's the time to look at "how do I move this adventure forward?"
Put another way, in an investigation setup like this I'd expect them - both in and out of character - to be proactive rather than reactive.