One tract: normal airway and correct tracheal intubation
Two tract: should consider esophageal intubation

For airway management and ventilation confirmation: use curvilinear probe
For airway evaluation and possible cricothyroid membrane: use linear probe

At least 3 important structures to be identified while you scanning the neck:
Trachea
Esophagus
Cricothyroid membrane

Location of your probe: transverse scan on the suprasternal notch

[Sonoanatomy] 氣管 Trachea

Double Endotracheal Tube (ETT) (Pig model)

Role of upper airway ultrasound in airway management

Airway ultrasound – 呼吸道超音波