In newer versions of Oracle Database it is not obvious where is the location of the alert file to investigate any potential issues.
You have a couple of ways to know where your Oracle alert file is located.
1. If the Oracle database is opened
You can use this query after connecting with: sqlplus / as sysdba
select * from v$diag_info where NAME = 'Diag Trace';
2. If the Oracle database is not opened
You can use adrci from the command line
# adrci ADR base = "/u01/app/oracle" adrci> show homes ADR Homes: diag/rdbms/yp001m1p/YP001P1 diag/rdbms/azamoa/azamoa1 diag/clients/user_oracle/host_2133002082_110 diag/tnslsnr/xm1iddb21/listener_dg adrci>
You can see there are several homes.
If you want to check the alert file for database YP001P1 then you should use the ADR base followed by the ADR home (from the output above) and then add the trace folder at the end
Once in that folder you can check the alert file
ls -lrt alert*