The tricky thing about nighttime mouth breathing is that it happens while you are unconscious, so most people who do it have no idea. But the signs are reliable once you know what to look for, and a couple of quick self-tests settle it. Here is how to know.

The morning signs

Your body leaves evidence. If several of these are true most mornings, you are very likely a nighttime mouth breather:

You wake with a dry, sticky mouth or a parched, scratchy throat. The single most reliable sign — air moving across an open mouth dries it out overnight. (Why mouth breathing dries you out.)
You wake with bad breath that clears once you are up and hydrated. (The saliva connection.)
You have morning headaches or wake up groggy despite enough hours in bed. (Mouth breathing and headaches.)
You snore, or your partner says you do.
Your lips are often chapped and you wake needing water.

The signs someone else notices

A partner is your best diagnostic tool here. Ask them — or check yourself if you nap near someone:

Do you sleep with your mouth open? The definitive tell.
Do you snore, especially on your back?
Do your lips part and your jaw hang slack as you fall asleep?

A phone photo or a quick video while you nap can confirm it in seconds.

The daytime self-tests

You can also check your default breathing while awake:

The awareness check. Several times a day, notice your mouth. Is it resting gently closed with your tongue on the roof of your mouth, or slightly open? Habitual mouth breathers tend to sit with the mouth slightly parted all day without realising it.
The five-minute nose test. Sit still and breathe through your nose only for five minutes. If it is comfortable, your nose works — so any nighttime mouth breathing is a jaw-dropping or habit issue. If it is a struggle, your nose is the bottleneck driving the mouth breathing. This single test also tells you which fix you need. (Why you breathe through your mouth at night.)
The tape-a-test-nap. With a clear nose, apply a small piece of mouth tape during a quiet awake period or a nap. If you are comfortable, you were probably fine nasally all along and just defaulting to your mouth. If you immediately feel you cannot get enough air, your nose needs opening first.

What to do once you know

If the signs point to yes, the fix depends on the cause you uncovered with the nose test:

Nose works, mouth drops open → keep the lips closed at night with mouth tape. (How to start.)
Nose is blocked → open it first with a nasal strip or treat the congestion. (How to stop mouth breathing at night.)
Not sure how bad it is → the free Titan Sleep Score quiz is a two-minute self-assessment.

When it is more than a habit

If your mouth breathing comes with loud, ragged snoring, gasping, or witnessed pauses in breathing plus daytime exhaustion, that can point to sleep apnea — a medical condition. Screen with STOP-BANG and see a doctor. (Apnea versus mouth breathing.)

The bottom line

You can tell if you are a mouth breather from the morning evidence (dry mouth, bad breath, headaches, snoring), a partner's observation, and the five-minute nose test — which doubles as a diagnosis of why. Once you know, the fix is simple: open the nose or close the mouth, whichever the test points to.