I would say that there are plenty of ways to find a business partners but knowing if it will be a good partnership takes more effort.

The best advice I can give in this situation is think about the business partnership like a romantic partnership. You want to be sure there is good chemistry, trust and communication. It is also great to make sure that you round out weaknesses by having that as a strength in the other partner.

What has worked? I myself have found a great business partner by testing the waters early on. We tried working on several small projects that didn't have a big financial risk or impact. This was to test how well do we work together, how communication techniques and how do we each handle challenges. If you are going to have problems with a potential business partner it will happen sooner or later regardless of the complexity of the project.

Also think about spending some time together outside of the business realm. Do you have hobbies that overlap? How do they act in a casual setting? Is it different than how they act in a business setting?

I think it is important to define roles early. What are you expecting from them? What can they expect from you? Who is handling specific job functions? Getting this stuff in writing is key (it can be flexible and change over time but this is a good first step).

I have experienced in past that partnerships only work when the partners have different and defined roles to perform in the organisation. This reduces the chances of any arguments. If every partner tries to work on similar tasks then work becomes very hard. Thus partners need to be patient, has business acumen and be able to understand extent of their roles as well,

Very carefully!