How do I preview draft posts?

Hi.

When I write a draft blog post, I used to be able to 'preview' it by logging into the blog on the front end.

Now I've switched off comments for my blog, as no-one ever comments anyway!

But now when I want to preview a new blog draft, I get a 400 error message. Please see attached screenshot.

How can I preview my posts before publishing?

Thanks.
Geoff