For linux:
$ strace sqlplus -L scott/tiger@orcl 2>&1| grep -i 'open.*tnsnames.ora'
shows something like this:
open("/opt/oracle/product/10.2.0/db_1/network/admin/tnsnames.ora",O_RDONLY)=7
Changing to
$ strace sqlplus -L scott/tiger@orcl 2>&1| grep -i 'tnsnames.ora'
will show all the file paths that are failing.