There is a standard and scientific  method to diagnose a pet’s disease which includes respectively:

1- Taking history: includes questions that your vet asks them from you like your pets  food, age, sex, spay or neuter and symptoms that you have seen .Therefor wrong  answers can be led to wrong diagnosis.

2-Examination: watching, palpation, physical exam and laboratory tests and special techniques      

 In watching your vet watches your pet’s body from head to tail to diagnose any obvious   change for example limping when it walks.

Palpation helps to vets to recognize any reaction to pain or recognize any macroscopic  abnormality especially when it is covered by hair.

 In physical exam a vet  exams body systems with a stethoscope like respiratory  or cardiovascular system and with a thermometer he or she measures the body temperature.  

To exam oral cavity or female genital system sedation is necessary.                         

3-Differential diagnosis: after above procedure there is a list of diseases which  can be involved. Sometimes diagnosis in this step will be finished and your vet on the basis of his or her knowledge , findings and experiences can diagnose your pet’s disease.Therefore it doesn’t need to go further.

4- Most often to approach an accurate diagnosis using laboratory tests like blood test or special  techniques such as radiography, biopsy,endoscopy are necessary.

 Although this is  standard approaching to diagnose a disease, sometimes your vet has to use   trial method to treat an unknown disease because laboratory tests and techniques can’t help  or the owner can’t afford the expense of techniques.