You can check whether the instance was started using spfile by the following query:

SELECT value FROM v$parameter WHERE name = 'spfile';

It will return the name of the spfile that was used during startup. If the result is NULL the spfile was not used during startup.

Or alternatively, you can query:

SELECT COUNT(*) FROM v$spparameter WHERE value IS NOT NULL;

Any non-zero result indicates the usage of spfile, a 0 as a result indicates the instance was started without spfile.

And no, the default for "ALTER SYSTEM SET ...." is "SCOPE=BOTH".