Demio doesn't have a specific feature for practicing/testing a webinar, but you can do so in a few ways:
If the webinar is going live in less than 24 hours, you may join the webinar room, where you will be put into what we call the backstage.
***Be careful not to hit the "Start Webinar" button! If you go live before the event, you won't be able to undo it and will have to create the webinar again and have the attendees register again.
If the webinar isn't going live for over 24 hours, you can create a test webinar and make it just like your upcoming live webinar. You'd just create a new event (just call it "Testing" or something) and set it to go live within 24 hours. Then you can start the webinar and mess around with everything, even go live, without worrying about messing up your upcoming webinar.