Nelio A/B Testing does not track admin users. Therefore, if you want to check whether Nelio A/B Testing is working or not, the first thing you have to do is log out from your account, use a different browser in which you're not logged in, or use Incognito Mode (or Private Browsing Mode).
Assuming you have an experiment running for the page or post http://example.com/foo, if the plugin is running and working properly you should see the following GET parameter*: nab=x, where x is the alternative loaded (0 is the original version, 1 is the first alternative, 2 is the second, and so on). For instance, http://example.com/foo?nab=2 should show the second alternative. If you want to load a different alternative, simply modify the value of the nab attribute and add nabforce to overwrite Nelio's randomly-assigned alternative (that is, http://example.com/foo?nab=2&nabforce).
* If you configured the plugin to hide A/B GET parameters, you won't see them (even if the plugin is working), because our tracking script will remove them once the alternative has been loaded.