Wordpress theme development for a big site

I am developing a theme for a site usign wordpress as a cms. The site has already more than 2000 posts. I am working locally on my computer and trying to simulate the post types etc here. Almost done. But I am very scared to upload and activate the new theme I have been developing locally.

What is the best way to test it and see everything is OK and be sure that I am on the safe side.

Thanks for your answers.