Upcoming project. I can't imagine that there could be such thing as a validated measure, but seeking references for a tool that has been used in the past. Thanks.
Mendl, M., Brooks, J., Basse, C., Burman, O., Paul, E., Blackwell, E., et al. (2010a). Dogs showing separation-related behaviour exhibit a 'pessimistic' cognitive bias. Current Biology, 20, R839-840. doi:10.1016/j.cub.2010.08.030
Mendl, M., Burman, O. H., & Paul, E. S. (2010b). An integrative and functional framework for the study of animal emotion and mood. Proceedings of the Royal Society B: Biological Sciences, 277, 2895-2904. doi:10.1098/rspb.2010.0303