I would suggest viewing their own property. If it isn't well maintained or is lacking in professional design, keep on looking.
We found there are many landscapers that merely mow grass, trim shrubs, and dig holes and stuff a seasonal plant and had to interview many to find a true landscape architect and designer. This is especially important for projects requiring grading, retaining walls, and hardscaping.
Be sure that they are properly licensed and registered with the state and require that their insurance company directly send you a certificate of insurance naming you as additional insured.