Ones you can trigger by yourself usually have a big orange question mark over the plot character's head. Other's need triggered by having a certain NPC's in your party.
My advice; Talk to everyone and attempt to open all doors. The system will say things like "You can't use this. Perhaps XXXX could." Where XXXX is the NPC you need in your party to trigger the quest.
If you want a breakdown of each quest one-by-one, hit here
http://www.gamebanshee.com/dungeonsi...alkthrough.php
Which is this site's Walkthrough area.
~Jessy