Going by audio alone, and with no context, you can't. Both should be accepted.
elle casse = ɛl kɑs
elles cassent = ɛl kɑs
This site is free. They are supporting multiple languages. Report it, move on and get any learning that doesn't happen in the questions here in the forum. Do you want to learn french or get more lingots? I want to learn french myself. We won't forget this lesson right? I'm sure everyone here was able to read and understand the response from PoupidouK in these comments in French. Good practice, and in context.