If you would like to understand the physics behind reflections and the methods and/or tricks that are sometimes used to avoid them, then Light: Science and Magic: An Introduction to Photographic Lighting explains everything in glorious detail.
Brief summary (from memory, bear with me!):
A good part of the book describes the concept of the "family of angles". These are the range of angles where any light sources will cast a visible reflection. Having all the light sources outside the family of angles will guarantee that there are absolutely no reflections, so when you achieve that you should be able to shoot through glass and the only thing from it that will get in your picture is the dirt in it.
This is hard to explain without diagrams, but basically the idea is that you take the cone determined by the field of view of your lens and bounce its boundaries as they hit the surface (the glass in this case). The area inside the bounced conic section is your family of angles. From the above it is clear that field of view, camera position and orientation, and surface position and orientation will all have influence in what is the family of angles for a given shot.
Since in your case you are dealing with natural light, instead of moving your lights outside the family of angles, you will have to move and orient your camera, relative to the glass surface, so that as much of the light source(s) fall outside the family of angles. Working with reflected natural light will make this a very challenging task, but nevertheless, I recommend that you read the book from start to end and master the concept, at least so that you can find how to minimize the issue.