That is the beauty of investing. If you think that fiat currency is more valuable than gold right now, you can hold paper. Some people think that gold is more valuable, including the Societe Generale: http://www.mineweb.com/mineweb/view/mineweb/en/page34?oid=94594&sn=Detail
Knowing something is a bubble gives you a big advantage, because you can then be nearly certain that a massive fall will eventually occur.
How does that give you an edge trading it? Once you factor out OTM asymmetrical option strategies there is little edge you gain trading the supposed bubble directionally. And even with options: nobody can guarantee you won't go broke paying for premium until the bubble finally bursts.
The Bubble rule kicks in when you begin to hear AM radio ads about putting your entire IRA into gold before it is too late. When you see the TV ads touting gold When the guy who polishes your shoes is talking about gold. Notice that when gold was trading 300 and under a couple of years ago, none of the three above was going on.
Because avg joe can still afford an ounce of gold if he wants it. That alone means that gold is not in bubble territory. bubble means avg people cant obtain it without carrying burdensome debt. Maybe if gold hits the upper thousands and 5 digits; then at that moment you can say gold might be in bubble territory. But who knows how the economic scenery would look like then.