Listeners are not bound to databases as such, in the sense that I have a running database with no listener at all running on the box. (Obviously connectivity from the outside world is not possible but you get the idea).

Similarly, a database can register with a listener on the *different* machine, ie, Machine "X" can accept connections for a database running on machine "Y", via a "hand off" style of mechanism.

If you want to see the listeners that are running you can use OS tools, eg ps

[oracle@vbgeneric ~]$ ps -ef | grep tns
oracle    1976     1  0 00:51 ?        00:00:00 /u01/app/oracle/product/12.2/db_2/bin/tnslsnr LISTENER122 -inherit
oracle    2101     1  0 00:51 ?        00:00:00 /u01/app/oracle/product/12.2/db_2/bin/tnslsnr LISTENER -inherit

or cluster tools (srvctl) if you are running under a cluster arrangement.

Looking at listener.ora on *all* of the ORACLE_HOME's that are installed on the machine would give you an idea of *potential* listeners on the machine, but as I said, that doesn't mean that they are started (or need to be).