Replication: How to query current segments allocation relative to "Wal keep segments"?

I'm maintaining async replication (streaming) between four database
servers arranged on 2 x 2.
How to  query current segments allocation relative to "Wal keep
segments" in each master server?
I want to add this query to Postbix in order to monitor if the "wal keep
segments" parameter is too short, like having an alert at 70%, for example.

Thanks,

Edson