The Xbox One isn't out until November, and unless you're lucky enough to be living near a travelling roadshow or consumer expo, you're going to have to wait until November to play one. Or...will you...
There is one other way. But I warn you. It is fraught with peril. And...American Idol washouts.
That way is to get a ticket on a "Celebrity Cruise", a cruise ship that offers performances by what they call "celebrities", but which are really a bunch of people who were on American Idol but didn't win. Like Mikalah Gordon. And Elliott Yamin. You know. "Celebrities".
Their fleet already has a ton of Xbox One consoles installed, it's claimed, in both permanent and mobile stations. Get on the boat and you can play Forza 5. Right now.
Is it worth it? For normal humans, no. Like I said, you will suffer. Cruise ships may flaunt luxury, but in truth they're often cannisters of suffering. Then there's the matter of paying for a ticket, and then putting up with the people who go on cruises.
But hey, there are fanboys, and then there are fanboys. If you're so keen to play a demo of an upcoming racing game that you'll trap yourself on a lawless hulk for days/weeks at a time, you're a more dedicated human than I, and my hat goes off to you.
UPDATE - Hilariously, turns out you don't even get to play it early. While touting the deal as an "exclusive partnership with Xbox", which "allows guests to be among first to enjoy Xbox One", as well as saying that their ships are "already are outfitted with dedicated Xbox spaces and mobile stations", the earliest cruises don't actually leave until December. Weeks after you could be playing an Xbox One from the comfort of your own home.