If there is possibility to test my service BEFORE I change the domain DNS? Domain which I have registered at another registrar.
I have service i.e. example.com hosted not here, but I want to move it to DH. But before I move my domain (change DNS at registrar) I want to test it i.e. at example.dreamhost.com or sth.
Is this possible?
Yep, there is a really good way to test a site before changing the DNS or otherwise “going live”. Check out the DH wiki article on viewing your site before DNS change.