Testing subdomain setup before transfering domain registration?

Is there any way to test a subdomain site before transfering the registration of the main domain?
I can test my main domain using the .dreamhosters.com redirect.
I’ve tried making a .dreamhosters.com redirect to my subdomain, but that doesn’t seem to work.

Note that both these sites are part of a Drupal installation.
When I try to use the subdomain redirect, i get the drupal page saying the site is down for maintence - but it’s for the main domain. And the main domain works fine.