In order to see what a visitor from a certain IP is seeing an additional parameter (mboxOverride.browserIp=188.8.131.52) needs to be appended to the end of the URL on a live running experiment .
However if you are using the preview links, the mboxOverride.browserIp technique is probably irrelevant as the preview links bypass the Audience-targeting. Hence it is needed to activate a real activity to see this. A safe way to activate an activity so that no-one else sees it is to include a special parameter in the Audience (e.g. qa=true) and then use it to load the page, (e.g. www.mywebsite.com?qa=true").