For me it would be sales and the "business" side of things. And perhaps some domain expertise. I can do design competently enough to make it work (I do most the design work at my current company despite nominally being the lead software engineer).
The problem I have is that I have no idea how one would go about evaluating someones capability at that skillset.
> The problem I have is that I have no idea how one would go about evaluating someones capability at that skillset.
I am a typical engineer type, and the few times I have correctly picked someone was when I was working directly with them - you can then tell real talent from their real world effects, even if it is not your own skill set.
Also one critical attribute when selecting a co-founder is integrity - if you can’t trust them then their other skills don’t matter. Working closely with someone gives you a chance to judge that - it is otherwise very difficult.
> The problem I have is that I have no idea how one would go about evaluating someones capability at that skillset.
I'm not pretending this is the best way to approach it (I'd approach it very differently, but I have the benefit of already having started my own company), but start with the obvious: ask them what they'd contribute and just let the questions flow naturally from there. If they can't sell themselves to you, they almost certainly can't sell themselves to investors, customers, and future employees.
The problem I have is that I have no idea how one would go about evaluating someones capability at that skillset.