You've met someone at a conference and are mighty impressed. Is it sufficient to take the big leap of making him co-founder? You might be looking for either technical or business co-founder, qualities to be looked for will overlap! A self-confident person committed and dedicated to work is always preferred over an irresponsible person. Such decisions of partnering with someone cannot be made by just first impression. It becomes easier to decide if you both have ever worked together and know each other well. But a colleague may not always be ready for quitting his job or taking such a great risk. So time comes to search outside your coworker-circle.
If you want to approach a person for partnering meet him face to face. Try to find out his areas of interests, not as sort of interview but in casual talks. Once you get to know his line of thought which you finding matching with you, put front one of your ideas. See how keen he looks toward it.
An interested person will have thoughts related to the idea running in his mind. Most likely he'll try to catch you up again sooner or later or even on chat. His inputs related to the idea will be a positive sign. A curious person will usually try to find products based on similar lines which are already available. Some very technical person might surprise you by presenting a working prototype of the idea the next time you meet!
Even if you want a business co-founder be sure that the fellow has sound technical knowledge. Ultimately business and technical work cannot be segregated altogether! Business co-founder should be good at communication, have the ability to say no when and where required. It becomes easier to judge if the person has prior record of business work worth appreciating. Just meeting and talking, without actual action, will hardly give you any idea about the person.
Initially doubts are sure be raised. Just ensure that even after repeated meetings and satisfactory clarifications the person is not stuck in the vicious sphere of doubts! All the best to find a co-founder whom you can trust!!