In order for the diagnosis of pica to be made, there must be a history of persistent consumption of a nonfood substance continuing for a minimum period of one month. Infants and toddlers are typically excluded from this diagnosis since mouthing objects is a normal developmental behavior at that age. Individuals with mental retardation who function at or below an approximate cognitive level of 18 months may also be exempt form this diagnosis.

Pica is most often diagnosed when a report of such behaviors can be provided by the patient or documented by another individual. In other cases, pica is diagnosed after studies have been performed to assess the presenting symptoms. For example, imaging studies ordered to assess severe gastrointestinal complaints may reveal intestinal blockage with an opaque substance; such a finding is suggestive of pica. Biopsy of intestinal contents can also reveal findings, such as parasitic infection, consistent with pica. Pica may also be suspected if abnormal levels of certain minerals or chemicals are detected in the blood.

Pica in pregnant women is sometimes diagnosed after childbirth because of a health problem in the newborn caused by the substance(s) ingested by the mother. In one instance reported in Chicago, a newborn girl was treated for lead poisoning caused by her mother's eating fragments of lead-glazed pottery during pregnancy.