The trap is probably the least documented in terms of procedures for play in RPG texts. It’ll have some damage and a save listed – done – let’s go out for Friday drinks!
Really no. I don’t have a source on this, but as I estimate it from seeing a range of modules and materials the key with traps is to have hints built into the room. Now there can actually be a range of hints, some of them making the trap nearly obvious – and harder hints that just barely tell the player where the trap is. But starting off I suggest with really fairly obvious hints and build up slowly over sessions from there.
So what’s a hint? Well if there’s a pit trap beneath a carpet, you could say the carpet is flaps in the wind a little. This might be a bit too subtle a hint, but many players might think that’s out of place – and it is for a regular carpet, but one strung across a pit and there are wind currents flowing through the dungeon is going to flap a little.
You’ll notice though that players home in on anything that gets extra description. Something gets described in more detail than it normally would? Players are all over it – they can’t help it, it’s normal, but it makes it a bit easy on them. Which is fine for when you’re starting out. Some players would not really think of poking the carpet with a stick unless they get a fair prompting before – they need the training wheels (and as they play over time they will get better and wont need them anymore). But when you are ready to increase difficulty, describe a few things in detail – like one or two other things apart from the trap. Or make the hint far away from the trap. A burn mark on a wall is classic for this – the trap isn’t on that wall, that trap is on the wall way on the other side and it shoots fire when triggered. The fire flies across the room and scorches the opposing wall. Players have to think inside the imaginary space and work it out.
But that’s the harder stuff – keep in mind as DM you can see the answer. A riddle always seems easy when you already know the answer. So keep in mind knowing the answer will make a trap seem much easier than it actually is.
Once your players are poking at traps with sticks, it’s a matter of using investigation checks or even disarm checks. If the way they describe their characters actions seems in any way like it could be more beneficial, give them advantage. Be generous with advantage at the start, allowing pretty much any extra effort to garner the bonus. In fact this is where many GMs go wrong – they think being really, really stingy with bonuses makes the game really, really real. And indeed it does, because reality rarely ever gives bonuses – but that’s the reason we’re playing a game, because reality sucks a lot of the time. So instead don’t try to be really, really real and instead use a difficulty curve that starts low with lots of encouragement and how hard it is to get advantage on a roll slowly increases over sessions and levels. I’d say to find the players level of skill in investigating traps and try and keep the difficulty just slightly higher than their skill, so as to provoke them to get better. That still means giving advantage/a bonus fairly often. Probably around half the time or forty five percent of the time. Give incentive to them to try more thoughtful and descriptive approaches to working out traps. If you dry up and don’t give any incentives they’ll give up trying to get better – if you give bonuses all the time then they don’t need to get any better.