Generally speaking, solid + opaque materials like wood or stone will seal, while nonsolid ones like materials meant for dirt decals won't. Some exceptions, for example caulk is a popular solid + invisible sealant, unlike i.e. all the nodraw and nodrawsolids which won't seal.
Another thing that helps a lot is to use a large grid size like 8 or 16 units sealing geometry. This way you can more easily align parts and see gaps. Details can be done with smaller brushes or with patches.
All of your detail pieces should be entities instead of worldspawn due to compiler reasons. This has the extra benefit that you can use the "All entities" filter to hide entities and only see your sealing worldspawn geometry.