To understand how the person across from you thinks, and in which category he/she is located, it's best to open all your senses and pay very close attention: to sight, hearing and feeling. You will likely notice that your attention is drawn mainly in one area more than in the other two.
For example, visual people will have a tendency to get our attention, because they are well dressed in bright colors; or their facial expressions and gestures will attract you, and get you to look at them. They have a tendency to stand face to face with direct eye contact.
They also tend to be humorous or funny and even risk making fun of others. In contrast they are not comfortable talking about highly emotional subjects or in conversations which are too serious.
To speak and act quickly and be frequently impatient with others is a strong sign for being visual, even if all visuals are not like that. They can be more or less shy or out going depending on the person.
As well you will notice that they access visual representations by looking up very often when they are talking.