Como saber si la instancia arrancó con PFILE o SPFILE?

Hay 3 maneras:

1) From V$SPPARAMETER View.

V$SPPARAMETER view lists the contents of the SPFILE. The view returns NULL values if a PFILE was used to start up the instance.

2) Using SHOW PARAMETER:

SQL> show parameter spfile;

This query returns the spfile name if the database was started with spfile.
It shows no value if the database is started with pfile.

 3) Using the alert.log file:

- When a pfile is used, it shows something similar to:
Starting ORACLE instance (normal)
...
Using parameter settings in server-side pfile D:\APP\MREHEEM\PRODUCT\11.2.0\DBHOME_4\DATABASE\INITORCL.ORA

- When an spfile is used, it shows something similar to:
Starting ORACLE instance (normal)
...
Using parameter settings in server-side spfile D:\APP\MREHEEM\PRODUCT\11.2.0\DBHOME_4\DATABASE\SPFILEORCL.ORA

Ref: How to Find Whether database Instance Started with PFILE or SPFILE? (Doc ID 444775.1)