The Sefaira Plugin will consider any SketchUp surface with an opacity of less than 100% to be a window or skylight, depending on its position. To check how Sefaira has interpreted your SketchUp model, activate the Entity Palette and click "Show Entity Types".
Note: It is possible in SketchUp to define opacity (or transparency) differently for two sides of a single surface. Only the transparency on the outside-facing surface is important for a correct interpretation by Sefaira.
For information regarding the specific properties of glazing applied in daylighting analysis (including transmissive specularity), see this article.