In oracle 9i and up spfile is the preferred way to start the instance.
In order to see if an instance has started with pfile or spfile as sys using sqlplus run :
select decode(count(*), 1, 'spfile', 'pfile' ) init_file_type
from v$spparameter whe rerownum=1 and isspecified='TRUE';
Also you can use the show parameter spfile.
In order to create a spfile from pfile the instance must be down.
create spfile from pfile; to use the default location which is $ORACLE_HOME/dbs on Unix and %ORACLE_HOME%/database on Windows.
In order to create the pfile if the instance has started with spfile run:
Create pfile from spfile;