WordPress as a CMS, transfering data from test URL to live site?

Hi,

I am by no means a DNS expert, but here is my problem.

I need to design a wordpress site for a customer, but I obviously don’t want to make it live until its done. They already have a site up on the URL that it will eventually be hosted on, what is my best plan of attack here? Can I test it on a sub directory or should I use a completely different url then change DNS setting afterward?

Thanks!