It can be difficult to tell how well a theme is optimized for SEO as some of the features are sometimes hidden. Highly optimized SEO is in the details and you can tell by how the code and features are described.
A well-designed theme would have clean code that search engines love. They would be built with SEO best practices which would focus on headings, Google rich snippets for posts and reviews, Google authorship, semantic HTML5 markup, CSS3, be responsive, etc. Some themes even give you some extra tools to help you customize and improve your SEO even further.