The objective of this study was to develop a screening instrument for pediatric physiotherapists to distinguish a symptomatic asymmetry in the clinical evaluation of young infants (age <6 months) with an asymmetric head posture. We chose two consensus methods, a two-round Delphi design and an expert meeting using nominal group technique, for reaching agreement about classification of diagnoses and clinical diagnostic criteria (CDC). Seventeen diagnoses with an expression of asymmetry with 69 matching CDC were assessed. In two Delphi rounds, six medical specialists and seven pediatric physiotherapists were polled anonymously on the classification, completeness, and relevance of the diagnoses and the CDC. Panel consistency in round 2, expressed as Cronbach's-α, was 0.89. In round 3, a face-to-face meeting with eight therapists, the previously selected diagnoses and CDC were prioritised, reduced to 10 diagnoses and 21 CDC, and completed with eight hard clinical signs (red flags). Finally, a differential diagnostic screening instrument, containing a classification scheme, the CDC for differential diagnostics, and a list of "red flags" was established on the basis of literature search and expert consensus. Cross-validity and reliability of the instrument will be investigated in future research.