After applying for B.Tech at IIIT Hyderabad, how can I check whether I'm eligible for admission or not?

You are eligible if you passed Class 12 with Maths, Physics, Chemistry and scored a minimum of 60% aggregate in PCM
Apply through one of the recognized channels like JEE Main, UGEE, DASA, Olympiad, or SPEC

How to check eligibility after applying:
IIIT Hyderabad will verify your application and documents. If you are eligible, they will shortlist you for the next phase (test/interview/counselling). If you are not eligible, you'll get an email or portal notification from them

After you submit your application, keep checking your email as well as the IIIT-H portal. They will inform you if you qualify or not.