How can I know if a self-designed structure I build will be structurally sound?
Safe loads are determined through engineering and experimentation. The experimentation is used to gather real-world data, which then is used as input to engineering processes, which then result in guidelines and building codes. 

This is all updated over time as new materials are introduced and new experience is gained.

When there's a situation that is non-standard, there are two common approaches to dealing with the situation. The first is to pay an engineer to provide guidance on an appropriate approach. The second is to simply overbuild. An experienced builder will know when it's okay to overbuild, and when you need an engineer.

To get back to your original question, it has no simple answers. It involves the materials used, how they are assembled, and what sort of forces they are going to be asked to deal with. If you can post more detail on your design, I can advise you on any deficiencies that I see, but I'm not a professional.