How can I automatically determine what browser features my code depends on?

The idea is to - limit our project to a set of browser features - ensure, that all browser we want to support provide the features we want to use - ensure our code only uses the features we use

The second task could be easily done using modernizer. But how could the third task be accomplished?

Motivation is to separate the test suite which tests business logic from testing browser support. Advantage is separation of concerns and faster runs for each test suite.