You can see whether autoextend is enabled for a datafile by querying as follows, depending on Oracle version...

In Oracle 7

select f.tablespace_name, f.file_name, e.maxextend, e.inc
from dba_data_files f, sys.filext$ e
where f.file_id = e.file#;

Note that if the sys.filext$ view does not exist, no datafiles are set to autoextend.

In Oracle 8,

select f.tablespace_name, f.file_name, f.maxbytes, f.increment_by
from dba_data_files f
where f.autoextensible = 'YES';

Note that the inc and increment_by columns are both in database blocks.

HTH

David.