For several reasons the importance of expertise is a sore subject for me these days -- most likely because my success has relied heavily on respecting expertise while I work in an industry that by and large does not. There is often a tendency to label people who have success as Rock Stars and then make the assertion that because someone is a Rock Star they can do anything. I don't see the data that bears this out.
I do, however, completely understand why it would be tempting to use the "Rock Star" standard.
It's not realistic to have expertise in more than a couple areas. The best you can hope for is expertise in some key areas and then a good filtering system to identify those who have expertise in the area you lack.
So what is a good filtering system? I am fond of the filter Jonathan Portes posits in a blog that answers the question of which economists to listen to for policy advise.
My answer to it is that policymakers and the public should listen to economists who fulfill two critera: first, they have made empirically testable predictions (conditional or unconditional – see Krugman here) that have proved, by and large, to be broadly consistent with the data; and second, they base those predictions on an analytic framework (not necessarily a formal model) that is persuasive. In other words, getting it right alone is not enough; it should be possible to show your workings – to explain why you got it right. Otherwise, your predictions may be interesting, but they tell you little about how to formulate policy.Translated more generally you want someone who 1. has delivered (either prediction, product, service) and 2. can explain to you how they did it. That can be a bit tricky in the product development / program management space because a lot of people have their hands in a successful or failed delivery. But, one mark of expertise is being able to do risk assessments beforehand that are essentially predictions of what could go wrong and it is useful to reference these when projects do go sideway.
Of course this is rather self serving because it tells you that I am very very good (ie have expertise) at Project Management in general and IT Systems Optimization specifically. On the otherhand, it also says my political predictions are horrible so there is that.