There are a number of different methodologies used to define a person's personality traits. An interview will uncover some of these, as will informal conversation, but the methods used in recruitment are usually facilitated by using a system, such as Myers Briggs or DISC. DISC is designed to assist people understand themselves, their behavioural style, and the impact they may have on others. The Myers-Briggs Type Indicator is designed to assess how a person reacts to their environment, both intellectually and attitudinally.
In the DISC system you are characterised into four types - D (Dominance: Direct & Decisive), I (Influence: Optimistic & Outgoing), S (Steadiness: Sympathetic & Cooperative), and C (Conscientiousness: Concerned, Cautious & Correct). Where you may be high in one area, you may score low in another. The testing asks the candidate to work through a series of 24 adjectives and asks them to describe the adjective that least describes them and the adjective that most describes them.
People who score very highly in one area of the DISC testing tend to stand out. For example if you have a 'high D' candidate it's likely that they'll walk in to the room and take control - they'll move quickly, they'll be self-assured, they'll be confident and chances are they'll run the interview for you.
Personality testing ranges in costs depending on the type of software that is used. As a guide, a basic test will cost around $100-150, although more in depth testing can involve a far greater investment.