It's possible to preview a creative without actually setting it live in a placement. The only requirement is that the site you want to preview the creative at has a zone invocation that accepts the same ad format as the ad you want to preview.
To preview a creative just visit the site and append the URL-parameter
preview with the ad id as value to trigger the preview. If the ad id don't exist, isn't live yet or don't match the size of a zone invocation on the site nothing will happen and the zone will display an ad based on its normal selection process.
Example: To preview the ad with id 123 on site http://bidtheatre.com that has a live placement of the correct size, construct and visit the following url, http://bidtheatre.com/?preview=123