In fact, I found very few researchers in career interest. The following reference may be of interest to you:

James Athanasou, (1990) "A FRAMEWORK FOR THE ASSESSMENT OF CAREER INTERESTS", International Journal of Career Management, Vol. 2 Iss: 2

Dear Kingsley:

I know of no scale to  measure career interest among adolescents. It is a common practice to search first in the NET. Have you already tried?

I wonder whether you are acquainted with Deci's and Ryan's theory of self- determination, which revolves around the idea of extrinsic versus extrinsic motivation. Generally speaking, one can say that intrinsically motivated individual are motivated by reasons intrinsic to their self or selves (e.g., intellectual curiosity, be creative, innovator, and the like). In contradistinction, extrinsically motivated individuals are motivated by reasons that reside out there or are external to the self  (e.g., to get money, be famous, be acclaimed, and the like). 

Accordingly, we can say that the more adolescents are intrinsically motivated the more they are interested in their career and, for example, dedicate a lot of time to it, do not give up when some problems arise, and the like. On the contrary, the more adolescents are extrinsically motivated the less they are interested in their career and, for example, dedicate little time to it, give up when some problems arise, and the like. Thus, adolescents' interest in their carrier can be classified, for example, as intrinsic versus extrinsic according to Deci's self-determination theory. I would prefer to say predominantly intrinsic or predominantly extrinsically because internal and external motivations can coexist in the same individual. 

I believe that adolescents' interest in their career can be assed through other parameters. However, there is certainly amassing evidence that shows that in their career it terms of effort, time, energy, and the like.

I attach to this message a PDF file I found in the NET addressing Deci's theory, extrinsic and intrinsic motivation, and how to assess them.

I hope  that this is of help to you.

Best regards.