I got intrigued to write this article, because I've been in the same company for eight months now, and seen the code that's been written before my time. And I think that some of the hires they had before me and my co-worker, was poorly hired.
But what should you do as a startup, it's mostly the visual parts that you can react on or comment on at least. You want a guy who is strong with the visual stuff, and this could be a checklist of things he/she should know.
Some sort of pre-processor sass, less, stylus, postcss these are the most mentionable BEM syntax, not a must, but a plus. Helps with the object-oriented CSS way of thinking knowledge of backwards compatibility (IE8+ support at least)OOCSS / SMACSS Methodologies
Most front-end developers are self-taught (autodidact), so education shouldn't be required if the personal skill checks off. In Denmark where I live, there isn't anything like a full front-end education, as a focus point. You can study software developer, but most of them are not that good front-enders they lack CSS skills. By that, I don't mean all, but the vast majority of the people I've met. Besides a good understanding of design has a value as well. The front-ender that can think for himself and interpret the designs in a good way, which is a very subjective matter.
But to find the great front-ender you need to have a developer that can teach others what that person learn. The better and well-thought group of developers you have the better the medium gets. So even the lousy developer with a hunger to learn will learn fast and grow into a good developer.
I don't think it's needed to know a specific framework, because if the developer is good enough, he will be able to pick up a new framework over a weekend. And right off the bat probably understand what it does.
Good/great front-enders are hard to find because there aren't any education where you learn all of it. The one you find needs to be able/willing to keep learning. And to teach others, what they learn.