[ladyjane]

Here:

https://www.acvs.org/small-animal/tracheal-collapse


From that site:

Diagnostics:
In general, the following tests are recommended to diagnose the degree of collapse, provide a clear picture of overall health and evaluate your pet:

bloodwork to look at overall health
chest x-rays (Figure 2, 3) may help with the diagnosis in some pets, and are useful for ruling out other conditions and looking at the size of the heart. Tracheal collapse is not always visible on regular x-rays.

fluoroscopy ̶ a moving x-ray ̶ will allow your primary care veterinarian to check the condition of your dog's trachea when it is breathing in and out (Figure 4). This is important since the size of the trachea can change depending on if your dog is breathing in or out.

endoscopy ̶ viewing the inside of the trachea with a fiber optic camera ̶ provides the best detail of the inside of the airway (Figure 5) and allows your veterinarian to take fluid samples for culture and analysis
echocardiogram ̶ an ultrasound of the hear ̶ to evaluate cardiac function