Here is my thought.
- Curriculums students took in secondary school, the performances.
- Open end tests.
- Essay or projects on specific STEM topics.
- interview by faculty (like how Oxbridge does it) focusing on essay, project and test
STEM shines in quantifiable measures, anything not quantifiable consider subjective. 3 and 4 should be indicators for aptitudes and interests.
EC should not be considered.