Usually when someone's saying something is overvalued, what they mean is that they think there is good reason to believe that what people are willing to pay for it now is significantly more than what they think people will be willing to pay for it at some undefined point in the future. Or, in a nutshell, that they think it's a fad.
I think it's generally a fairly safe bet to assume that, when there's a buying rush on something, the market price will end up being quite a bit higher than what it will be once things start to stabilize. That's a fairly intuitive implication of supply and demand, and I think the only real way around it is if there's some other force that's constantly working to push the price up. For example, the price of land tends to appreciate because the supply of land is approximately constant, but the number of people who want land is always growing.
Bitcoin maybe has that with its deflationary design. But a counterweight to that feature is that nobody really needs Bitcoin for anything. So, like land, the supply is pinned, but, unlike land, it's entirely possible for demand to plummet all the way to zero.