I assume that you refer to reflected scenery..... there are two articles in my book concerning this. Methods include:
1: You either place a very large photo of sky/clouds in a transparent material used as the glass - preferably slightly rippled in Photoshop to emulate actual glasspanels in frames.
2: Make a panel behind the camera with an image of the scene you wish to reflect on it. Adjust the panel according to what you want to see.
Dwight Atkinson