Do they list their clients on their website?
Good consultants will not be afraid to list their successes on their website. They will proudly display the websites that they have ranked.
Do they offer other services?
Web design and coding is an integral part of SEO. The best web companies will list services that are additional to SEO. If you do not know how a website pieces together, then you don't really know how a search engine will see a website.
Is their English good?
The best SEO consultants will be able to twist and bend the English language to their will. They will be able to create words that sell. This is the whole point of SEO after all.
Do they already rank for particular keywords?
If they sent you an email, google their name. See if they already rank for a set of keywords. The best SEO companies will already be number one for keywords that count.