I mean you can witch hunt if you want but there's really no way of knowing. Especially in alliance raids where new comers are likely to try and follow the crowd.

For example, during Althyk and Nymeia you're going to see a lot of people doing whatever the up coming mechanic is the moment they see the cards, even though the cast bar for that mechanic hasn't been cast yet. All it takes is one player to panic and start doing the mechanic early and suddenly half the raid is following them.