Build the partnership through shared work
A cofounder relationship needs evidence of trust, not only a useful combination of skills.
Work with people before you make a durable commitment where possible. Observe how they respond to pressure, rejection, mistakes, and disagreement. Notice whether they finish difficult work and tell you uncomfortable facts. Enthusiasm for the first idea does not answer these questions.
Complementary skills can help, but they cannot replace respect or shared responsibility. People with similar skills can also form a strong team. They must understand what the company lacks and remain willing to learn. Choose for the relationship and the work, not for a complete-looking team slide.
A cofounder provides more than specialist labor. You will share uncertainty and decisions that affect your lives. Mutual admiration and emotional support can help the relationship survive a change in direction. The first product may disappear while the need to work together remains.
Do not postpone all work while searching for an ideal partner. A solo start is possible, although it can carry greater personal strain. Small projects and useful collaboration can create the relationships that a formal search does not. Explain the commitment honestly when you invite someone to join.