If you were to create a test that measures someone's linguistic capabilities, what would this test include?

Test cases would include , test forGrammar, Vocabulary, phonotics and tests for gender and basic tests on communication. 

Based on the language's complexity and the expected situation of usage (Its Managerial, Its middle men, Marketing or Sales )some standard responses can be tested. 

If the requirement would involve excessive reading or writing , then tests can be alterted accordingly

i would follow the IELTS and TOEFL standard test .

reading 

listening 

speaking

writing