Properly describing enemies: how much detail to provide?

Question

Faced with a group of enemies, players in our group tend to ask for enemy descriptions in some detail--the kind of armour/clothing they're wearing (to get an idea of who are the casters, the nimble combatants, the heavy-plated fighters), the state of their gear (does the armour look plain/ornamented; rusty/well cared for; is it leather/chainshirt/full plate), and so on. The idea, of course, is to try to identify the biggest threats--especially casters, which, afaik, provide little to no visual cues re: their level of power.

So, a few questions:

1. In general, how much info should I give about the threat posed by enemies (if any)?
2. Concerning casters (or any enemy whose level of threat is hard to discern based on appearance), should I provide hints to all players about the formers' the threat they are facing?
3. If not, can casters in the party make Arcana checks to have an idea of the threat posed by a caster (i.e., more powerful than me, WAY more powerful than me, cockroach running blind)?

Any ideas concerning 3 would be especially welcome.

Answer 1

The general rule here is that players should get the information that should be apparent to their characters - in the amount of time allotted. So if they get to size up the group for a minute before the combat, that's a different thing than if the other group kicks down a door and suddenly you're in initiative.

1. Immediate Impressions - Players should definitely get information on obvious weapons/armor/demeanor of their foes. I've often been frustrated by GMs that omit obvious "need to know" information ("What do you mean, they have wings? You could have mentioned that three rounds ago..."). Some amount of power level may be able to be intuited from that - just like in games like World of Warcraft, when you see a low level person they have normal clothes and armor and when they are high level they have everything with glowing purple ground effect lighting and magical stuff whizzing around their heads. The average first level wizard in a 1 gp robe is going to look different from a high level one with 5 ioun stones and a Robe of the Archmagi. You can see the Archmage in the NPC section of the MM for an example of what a high-level magic-user might look like in 5e. BUT, this can be deceptive, and a level 18 wizard dressed incognito in dirty leathers looks just like any other guy.

2. Some Time To Observe - There is not (and should not be) any objective power that gives you the exact threat level of opponents - magical or otherwise. But if you have time to interact with them, you can perhaps get clues. If you watch a skilled fighter fight, or watch a high level wizard cast spells, you can probably get a sense for their skill level. When you see Tony Jaa take down some opponent you get a pretty clear sense that he didn't just get in a lucky 5 hp hit, but that he could kill a near infinite number of you-s. Taking actions to scope an opponent out carefully, looking for telltales of how he stores his gear, how he moves, etc. can perhaps provoke a skill check that could reveal "he's a noob," "he seems skilled and at your level," or "he seems pretty bad ass." If you have a very long time to interact, like at a ball or something, then you could even use social skills to try to pull out further hints. "Thanks for the flaming drink! Well, that reminds me of the time I used Time Stop to take out a balor..."

If the players are just trying to hedge their risk by knowing exactly what the power is of everyone they kick down a door on - tough. "He's a bearded human in platemail and has a silver sword ROLL FOR INITIATIVE!" If they are realistically taking their time to assess opponents, in either short time spends (combat actions) or long time spends (espionage) then they can find out more.

Answer 2

Tell Them What They See at a Glance

It is very common to "size" up your opponent. Give them their "at a glance" information. When they begin their "glaring at a person" questions - have that person react. If I noticed someone staring at me and looking me up and down, I would probably say something to the effect of, "Excuse me?" If I was hostile to them I would probably say, "What the !@#@! are you looking at?"

There are skills to give that sort of information, as well as spells. Use your ability as a DM to control the amount of time allotted. If they are literally taking up too much time trying to figure out every bit of information possible about an opponent, then tell them to roll initiative and get it started. As soon as combat starts, they will very quickly figure out who the biggest threat is.

Take charge of your campaign. Are you the Dungeon Master or a Customer Service Hotline? Unless they are using specific methods to gain information, keep it to what they can gather at a glance and at a moment's notice. If they continue, then maybe that set of Full Plate is disguised as a Robe. Maybe that Robe is disguised as Leather Armor. And maybe once in while it is the enemy asking questions about the players.

Answer 3

Physical descriptions should be immediately apparent and related.

5e's bounded accuracy system applies to both PCs and monsters and while some creatures have "natural armor" which moves their AC above the strict 10+DEX almost everyone else will have their AC modified by Armor as well. PCs should easily be able to see if one enemy is wearing plate and another leather and know that the plate wearing enemy is probably going to be tougher to hit but also have worse DEX saves vs spells. I would also describe natural armor as appropriate for the monster to get across they have a higher AC. Spiders have "a gleaming chitin" whereas a dragonling would be covered in "hard scales" etc.

Likewise if there is a wizard you should from the statblock be able to describe that he/she is wearing robes and has a staff (or whatever is appropriate). That alone should signal to the players MAGIC USER!

Discovering a magic user's power level is a tricker tid-bit

RAW there is no mechanical set way to do this. You could have the PC(s) make an arcana check but I would have it be an opposed check for the magic user to try to shield their true power or bluff a higher power (whatever makes sense for the NPC enemy). Honestly though I would not allow detection like this or at the very least if I did then I would have it cut both ways. The best way for them to realize a magic user's strength would be for a magic user in the party to recognize the power level of a spell as it is cast.