You will need a conductor or other bystander for this method and will also need to be playing in a group.
After the group has played through a section of music the conductor will nominate one member. That member will then need to tell the conductor what another member of the group was playing.
Play through the section of music again and this time the nominated member will have to identify what a different instrument was playing.
If you are getting good at this you can try playing through a piece without nominating someone beforehand – that way everyone has to listen to everyone else for fear of being nominated.
You will find that all members of the ensemble start to listen to each other a lot more clearly for fear of being nominated!