How to know if I can use Trade In?

I would start here with the trade-in FAQs:

Apple Trade In - Apple

(scroll to near the bottom of the page)

Also read the small-font note at the bottom of the page. It says that trade-in may not be available in all locations. Presumably, if you use the estimator near the top of that web page, Apple can see you approx location from your IP address and tell you if you are eligible.

Beyond that, your need to ask Apple and they will not see your question in this user-to-user forum setting.