Rumination disorder is most of the time diagnosed by a pediatric gastroenterologist. Sometimes endoscopy is done but not always. 
Diagnosis is outlined by Rome IV as follows,
B4. RUMINATION SYNDROME
Diagnostic criteria* Must include all of the following:
*Criteria fulfilled for the last 3 months with symptom onset at least 6 months prior to diagnosis
Supportive criteria
				
			Diagnosis is outlined by Rome IV as follows,
B4. RUMINATION SYNDROME
Diagnostic criteria* Must include all of the following:
- Persistent or recurrent regurgitation of recently ingested food into the mouth with subsequent spitting or remastication and swallowing
 - Regurgitation is not preceded by retching
 
*Criteria fulfilled for the last 3 months with symptom onset at least 6 months prior to diagnosis
Supportive criteria
- Effortless regurgitation events are usually not preceded by nausea
 - Regurgitant contains recognizable food which may have a pleasant taste
 - The process tends to cease when the regurgitated material becomes acidic