Hi Angella

Nutrition knowledge, attitudes and practices (KAP) surveys are widely used to assess nutrition knowledge in a variety of population groups (including adolescents) in a variety of settings. The FAO has published a reference guide and tools for conducting such surveys here: Guidelines for assessing nutrition-related knowledge, attitudes and practices (fao.org) and there are similar guidelines/tools available here:The KAP Survey Model (SPRING)

Various questionnaires have also been used to assess nutrition knowledge in individual research projects, tailored according to the context. There are many examples from peer-reviewed publications, but this review of the literature summarising nutritional status of school-age children and adolescents in low- and middle-income countries across seven global regions may be a good place to start to identify existing research and tools used to assess adolescent diet and determinants (including nutrition knowledge): Nutritional status of school-age children and adolescents in LMICs (PHN). 

Hope that helps.

Stephanie