How do we know that, parallelism is enabled on a table?

SELECT TABLE_NAME, DEGREE FROM USER_TABLES WHERE TABLE_NAME='YOUR_TABLE_NAME';

I encourage you to get familiar with the Oracle documentation on Application Development so you can get the most out of Parallel Processing.

The 10g Oracle Database Warehousing guide has a chapter on Using Parallel Execution here: link

Another helpful page is the Catalog Views / Dictionary Views reference.