This paper provides a nice checklist of itmes you should look for in a study establishing content validity ofr a patient-reported outcome measure.
C.B. Terwee, C.A.C. Prinsen, A. Chiarotto, M.J. Westerman, D.L. Patrick, J. Alonso, L.M. Bouter, H.C.W. deVet, L.B. Mokkink. COSMIN methodology for evaluating the content validity of patient-reported outcome measures: a Delphi study. Quality of Life Research, 2018, 27, 1159-1170. DOI: 10.1007/s11136-018-1829-0. Available in html format or pdf format.