I am thinking of signing up as a reseller.

However i don't have a clue on how to do this...I am offering 3rd level domain such as newuser.mydomain.com to new customers.
How can i validate automatically whether a 3rd level domain
exists or not ?

For example the new customer need to know that newuser.mydomain.com is already taken but newuser2.mydomain.com is available.

Is there is any scripts to process this ?
Does C Panel deal with this ?