Background process messages can be traced using:
ALTER SESSION SET EVENTS 'immediate trace name bg_messages level level';
where level identifies the background process (+1)
For example to find the level of SMON, first identify the index number using the following query:
SELECT indx + 1 FROM x$ksbdp WHERE ksbdpnam = 'SMON';
The following ORADEBUG command has the same effect:
ORADEBUG DUMP BG_MESSAGES level