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I had an interesting thing happen in my last session of Dungeons and Dragons 5th edition.
My players are in a dragon heavy adventure.
A player tried to do a perception check to detect the scent of lilacs. Normally whenever a character wants to do a perception check I just tell them to go for it, though this time I asked them why.
It seems that he heard somewhere that a dragon in human form smells of lilacs.

I had never heard of this before. Clearly there are at least 3 relevant questions:

1. Do dragons in human form have a smell that they can be identified by?


2. Are there actually any dragons in human form in the room? (This one I know the answer to)


3. Even if dragons in human form do have a smell, then how would the character know this? You would think that dragons would want to keep this a secret.

Have any of you heard of anything like this before?

I did not allow the check because he couldn't show how his character would have come across this knowledge. Though I was tempted to have him preform the check only to notice that the room was lined with lilacs.

Their are a few ways this can play out,

1. If there are no dragons, then they can't find one no matter how hard they look.


2. There is someone who smells of lilacs, though if they are or are not a dragon is another matter entirely.


3. There is no way that they should have heard this, so I don't allow the check.

Disclaimer: This answer was written before the addition of the 5e tag. The content below may not be applicable to that specific edition of D&D.

D&D 3.5's Draconomicon actually has several examples of dragons having specific smells. It seems to vary by dragonflight color:

Black - Smells like rotten vegetation and foul water.

Blue - Smells like ozone and sand when upset.

Brass - Smells like hot metal and desert sand.

Bronze - The smell of the sea.

Copper - a "stony" odor.

Gold - Safron and incense.

Green - Chlorine.

Red - Sulfur and pumice.

Silver - The scent of plot devices rain.

White - Couldn't find anything about its scent.

From the classic species, none of them has the smell of lilac. More so, from what is possible to see, it seems that every species of dragon has a different scent, so I would say that no - in general, dragons don't smell like lilac.

Keep in mind: Those are the scents for their regular, dragon form. I wasn't able to find anything regarding their scent either changing or remaining the same when turning into a human.

As DM, you can decide whether or not dragons have a distinctive smell or not; even if it is written in a book somewhere, you are free to change that.

As DM, you can decide how to determine whether or not a given character would know whether or not a dragon has a distinctive smell, and if so what that smell is. I suggest that, in this case, your best course of action is to treat the suggestion that dragons have a particular smell as the player asking “does my character know of any way to determine a dragon in disguise?” and then ask for an Intelligence check. Which Intelligence check is unclear—unlike previous editions, D&D 5e does not define which Intelligence check covers knowing things about dragons. In previous editions, that would be Arcana, but 5e defines Arcana as dealing with creatures from other planes, so perhaps Nature would be more appropriate. Personally, I would probably let players use either, since dragons are natural creatures that yet have a great deal of arcane relevance, so people trained in either might well know things about them.

And then, as DM, you can decide how hard whatever “tell” they know about is to actually detect. This will probably be a Perception check, since it’s all about recognizing something in your environment, but the DC would be up to you (you could probably make arguments for other skills but most of those are probably not going to be things you can just do inconspicuously when you find yourself in a room of strangers.

Lore

Do dragons in human form have a smell that they can be identified by?

As far as I am aware, there is no lore in any version of Dungeons and Dragons I've played that indicate a dragon polymorphed or not has a distinctive smell. More specifically, I'm am certain there isn't any game mechanics in 5th edition that allows a polymorphed dragon to be identified by smell.

Game Settings

The game system is setting independent, even if the setting your player has read about did have lilac or lavender scented dragons doesn't mean that your setting has to. Currently, the 5e rules were written with Forgotten Realms in mind; but expansions have made with Ravnica, Ebberon and Barovia (in Ravenloft)

RAW

Even if dragons in human form do have a smell, then how would the character know this?

If it did (and even not), the DM could ask the character have to make a history (Do you know the lore), Nature (Do you understand the nature of Dragon anatomy) or Arcana (Does draconic magic have scent) check (your choice) if they think the character might know this information. They would do this before they do the perception check you ask for.

Note: Players shouldn't ask to roll checks. They tell you what they want to do, and you tell them if a check is necessary.

Fun with Meta

I personally, would love using the meta belief to my advantage as a DM. "Roll a history check." They roll poorly, below the DC you set on that information. "Yes, you are positively convinced that dragons smell like lilac." They make a perception check, to see if the person smells, and succeeds.

• If they aren't a dragon, I make them smell like lilac, so the character thinks they are.


• If they are a dragon, I make them smell of overly burnt sandalwood incense.