Just go down the standard list that you would with any professional. Education, past experiences, favorite style, porfolio, and personality. For most cash strapped homeowners, it's also important that a master plan be developed that can be implemented in stages over the life of the home, so asking about projects that took that approach would be helpful for you to hear about.

IIRC you are in a development with lots of design restrictions. Make sure to have a copy of those and make sure the LA is willing to work within those parameters.

Also ask if they recommend specific plants or plant types. Get references and determine if s/he will listen to you and your limits. We paid a landscape architect to design our landscape. He is well known in our area and has been in business for decades. He returned with a plan that contained exactly what we told him we would not do and did not want (an addition to our driveway to make it circular, a change in the size of our deck and the location of stairs). His plan contained no specifics on plants to be used. It was disappointing overall.

We did use one element of his plan for some other hardscaping. A few years later we went to a nursery that will design a landscape plan. That plan ended up being free because of what we spent at the nursery.

Think about hardscape vs. plantings. Think about maintenance and native plants with common pests. Think about irrigation. think about ultimate height/size of plantings vs. growth rates (fast vs. slow). Think about how much color vs. green you want. Think about perennials and bushes vs. annuals. Think about how many plants you need, not for the first year, but after they grow in for the 5th and 10th years. So many landscapes look nice the first two years, then completely overgrown thereafter. Err on the side of more blank space in the early years...they will fill in naturally.

And like any other supplier, compare prices, look at their prior work and get references.

OMG thanks everyone!

Pixie- we don't have a lot of requirements on landscape so that shouldn't be an issue.

Yes, hardscape also needs to be considered... Permanent driveway location, walkway to front door, backyard deck and BBQ area... There's A LOT we want to do!

Would a typical landscaping plan include those hardscape elements as well, or would most LAs consider that an add-on?

Thanks again everyone for your input. I feel better prepared now :)

A good plan will necessarily include both hardscape and plantings as you want them to work together to build the outdoor rooms and requirements you have, as well as taking the design and architecture of the house into consideration. Other issues like handling water runoff and slopes to protect the foundation, burying wires for outdoor lighting (I recommend running a pipe under steps or walkways for future wiring needs) should all be taken into consideration when the planning is done.

You might want to do a web search including key words of your question, along with "gardenweb" and "Landscape Design forum". While the Landscape Design forum currently has fewer professionals now, it once had quite a number of helpful (though not always tactful) pros and I learned a lot there.

Since this is a design/style thing, not only do you want to find someone who listens and has the appropriate training, but you want someone whose style fits what you want. So get references and not only talk to the reference for the same type of thing you would ask on any job, but either look at some of their work in person or look at their portfolio. There are also differences between landscape architects, landscape designers, and engineers, and depending on what you want done, different folks will bring different levels and types of expertise and training to the job.

Some general things to think about before you have your meeting(s):
Make a list of what you want in your yard and how you want to use it, ignoring what is currently there and not there. Are you a gardener and want to grow ornamentals or vegetables? Will you do your own maintenance or will you hire real gardeners or a mow, blow, and go service? Do you have kids or pets, and if so what are their needs? What is the traffic flow in your yard and do you want to change some of this? Do you like to entertain, and for what size groups? Do you like sitting out in your yard? Do you need to have any storage (trash cans, compost, lawn mower, etc.)? What issues and problems are there, such as lack of an afternoon shaded seating area or water flow or views you want to borrow or others you want to block, etc. What is your budget, and will you be doing this all at once or in stages, will you be doing the work or do you plan to hire out the work?

A good book on home landscaping will probably have more of these questions to answer. Once you have thought through and answered some of these questions, you will be more ready to evaluate whether your landscape architect is right for you.
You might also want to sketch out ideas, look at other folks' yards, look at other yards and gardens on the web or in books from the library so you can bring things that appeal to the meeting. In general, hardscape needs to be done first, but is also the most expensive part of the job, followed by any soil work. The plants will be the last thing added.