Did a SQL Server instance ever participate in replication? How can I tell whether a SQL Server instance ever had replication in use or not? I'm debugging an issue with a 3rd party software. This software (Red Gate SQL Backup) reports the distribution database to be missing. The company has asked to find out whether this server has ever used replication or not. How can I find out?

Replication does not seem to be installed and the distribution database is not present. None of the databases use replication.
I'm not sure if you'll be able to reliable tell if the server has ever used replication or not.  For example, if someone drops replication properly, there will be no trace left behind.