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.

HTML/CSS

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

Javascript

Being able to read others code, understand it, and debug it. Being able to write a javascript library. Know how to build a small application; It could be a simple version of your website/product or maybe just a module of your product (you need to serve the data). Knowledge of REST api's and how to consume them. The knowledge of some sort of MV* framework being able to write tests, depending on how much you have to spend, I don't think it's crucial but that depends on your business goals. Version control like Git/SVN is a must or should be. If you don't use that in your organisation you should start using it today. Personal skills Have improved his/her skills, and have the passion for keeping developing his/her skills. It's key to this position because it changes so quickly that if you don't develop your skills, you can't keep up with the business.

Education

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.

What else?

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.