So I reached for my trusty IDE (MS Visual Studio at the time) and coded me some trap descriptions - including defining the trigger mechanism and what the characters might notice - when and if they detect the trap.
There are many different types of traps, but since I was coding it I started by grouping traps by type, mostly so that I could weight the occurrence of each type within the dungeon and by challenge rating (3.x remember).
Trap Type - D100
01-05 Spells - Traps that involve magical effects
06-20 Weapons - Traps that swing a blade or club or fire a missile
21-40 Deadfalls - Traps that drop something
41-65 Channeling - Traps that prevent/require the characters to move along certain paths. (Chutes, Portcullises)
66-00 Pit traps - Word of advice from my players "Don't ask the cleric to open the chest. She can't heal herself afterwards."
For each type I then determine what the characters might notice about the trap itself. Using a portcullis for an example:
1. They see a row of evenly spaced chips in the floor
2. They spot a thin gap in the ceiling
4-6 They spot the trigger.
1. They see a small raised area of the floor about 7cm square. [Stepping on the lever triggers the trap]
2. They feel the floor start to give under their foot. [Stepping on the lever triggers the trap]
3. They see a thin cord stretched across the floor. [Pulling a tripwire triggers the trap]
4. They see a thin wire stretched across the floor. [Pulling a tripwire triggers the trap]
5. They feel a resistance at ankle level. [Pulling a tripwire triggers the trap]
6. They spot a creature that looks like a greyish brown fried egg on the wall. [It's a Trap Limpet - a magical construct that can trigger and reset a trap. It will be a forthcoming Beastiary entry.]
7. As they lift the container, they feel it being pushed higher by an unseen force. [Moving the container triggers the trap]
8. The container rests on a small pedestal. [Moving the container triggers the trap]
Oh, and despite my categorizing it as 'Channeling', I did kill a character with one once, when the thief failed to find it, it dropped on the guy fifth in line behind him. Cause of death - multiple BIG stab wounds.
I'll walk through more traps, triggers and what they see in further posts. If you have suggestions to add for triggers and observations, please leave them in the comments.