What are ways to assess employability of workers?
Measures of person-job fit and/or person-organization fit. The following rather randomly assembled references are intended as links to some questionnaires. 

References

Chang, H.-T., Chi, N.-W., &amp; Chuang, A. (2010). Exploring the moderating roles of perceived person-Job fit and person-organisation fit on the relationship between training investment and knowledge workers’ turnover intentions. Applied Psychology: An International Review, 59(4), 566–593.

Kooij, D. T. A. M., van Woerkom, M., Wilkenloh, J., Dorenbosch, L., &amp; Denissen, J. J. A. (2017). Job crafting towards strengths and interests: The effects of a job crafting intervention on person–job fit and the role of age. Journal of Applied Psychology, 102(6), 971–981. https://doi.org/10.1037/apl0000194

Piasentin, K. A., &amp; Chapman, D. S. (2007). Perceived similarity and complementarity as predictors of subjective person-organization fit. Journal of Occupational and Organizational Psychology, 80(2), 341–354. https://doi.org/10.1348/096317906X115453

Saks, A. M., &amp; Ashforth, B. E. (1997). A longitudinal investigation of the relationships between job information sources, applicant perceptions of fit, and work outcomes. Personnel Psychology, 50(2), 395–426. https://doi.org/10.1111/j.1744-6570.1997.tb00913.x

Wheeler, A. R., Gallagher, V. C., Brouer, R. L., &amp; Sablynski, C. J. (2007). When person-organization (mis)fit and (dis)satisfaction lead to turnover: The moderating role of perceived job mobility. Journal of Managerial Psychology, 22(2), 203–219. https://doi.org/10.1108/02683940710726447