Your main concern should not be the areas of practice, because those will be similar for a startup lawyer as compared to a lawyer that deals with more established companies. (However, I've put together a list of those below.) Instead, you need to look at how they've organized their practice around startups. Here are some good indicators:
(A) What do their rates look like? Do they offer flat-rate plans for set tasks? (Ex: a startup package, an equity incentive plan, a seed financing round) Do they demand big up-front retainers? (retainers only really make sense when they're going to do a big chunk of work and there's a risk that you won't have the money to pay when it's all done.)
(B) Do they take equity for services? Startup lawyers sometimes will take a small equity position for a portion of their fees if you ask.
(C) Do they know people in the local startup community -- can they point you to people or organizations that will help?
(D) Do they get the culture? See who they follow on twitter.
Anyway, here are the areas of practice that a startup attorney should be in:
(1) Corporate/securities law: forming corporations, issuing shares to founders, advising boards, private offerings of debt and equity, asset sales and mergers
(2) employment law: dealing with independent contractors and employees, employment agreements, equity compensation plans, etc....
(3) Contracting/Licensing: this can be domain-specific -- licensing a bunch of biotech patents is a different animal than software licensing.
(4) tax law: Not a huge issue, but need to understand tax consequences of various corporate transactions and compensation schemes. If the engagement letter says "I don't give tax advice," you probably have the wrong lawyer.
Here are some things that you don't need in your startup lawyer:
A. Patent law. If you need a good patent lawyer, your startup lawyer ought to be able to get you in touch with one. But, patent law is a speciality, and patent folks generally don't have experience with 1-4, above.
B. Litigation. Litigation is time-consuming and nuanced. If your attorney spends any substantial time litigating, chances are that he doesn't really have the skills in those other areas. Deal lawyers tend to be collaborative; litigators tend to be combative.
C. Real Estate. You won't be buying a house or entering into complicated lease transactions. It's helpful to have somebody who understands leases, but you probably don't want somebody who spends all his time doing real estate work, unless your startup is somehow focused on real estate.