Shooting in front of windows can be challenging. I'm not talking about using a window as your source light, e.g., window-lit portraits. I mean when windows are your backgrounds.
Before shooting in front of windows or any highly reflective surface, it's a good idea to understand the Laws of Reflection, commonly called Angle of Reflectance as it applies to photography.
In the first photo I'm providing, I'm shooting outdoors in harsh daylight with two sets of windows in the background: One directly behind Christina, the model, and the other on the wall behind the background glass. I was using one monolight modified with a 5' Photoflex Ocotodome: A rather large modifier that would reflect quite plainly and obviously if I allowed it to.
Shooting in front of glass windows, especially if you're using strobes, requires you to be keenly observant. First, you're looking for specular reflections caused by your strobe when it fires. If you're not using a modeling light, which helps reveals those reflections, you might need to take a few test shots to determine if the strobe is reflecting in the window or causing a flare. Often, the strobe and its modifier and stand are readily seen without firing the strobe. In pre-digital days, you weren't able to chimp the image so, in many ways, your understanding of reflections and how they appear, as well as your powers of observation, were even more important.
Reflections follow laws of physics. Light reflects at an angle equal to the angle it originated from. As an example, if your strobe is placed at a 45º angle in front of and to the left of your model, it will be reflected at a 45º angle to the right of your model. What does this tell you? If you don't want to see the reflection, place yourself on the same side of the model as the light that will reflect. Reflections, BTW, fall into two categories: Specular and diffuse. Sometimes, you might want to see reflections in your shots, e.g., when you want the model reflected or when you want diffuse reflections adding highlight to the BG in order to grab the viewers' attention.
You also need to be on the lookout for people or things that might be reflected in the window. You, the shooter, might be reflected. Others, behind you, might be reflected. Again, pay attention. Be observant. Search in your viewfinder for people or things you don't want reflected in your image.
I didn't spend much time shooting Christina outside as I kept fighting stray sunlight striking Christina in places I didn't want to see highlights. The sun was high in the sky which made using it as natural back light near impossible. In the first pic (above) you can see the sunlight hitting her arm above the elbow and on the back of her calves and on her left shoe. I didn't like that. Yeah, the way the sun highlighted her hair was cool but positioning her so that the sunlight reflected off her hair and not on other parts of her body became increasingly difficult. I decided to move indoors as shown in the the second photo.
Fortunately, in the spot inside the home where I decided to shoot more images of Christina, there was a skylight overhead that would provide hair highlights. (There was nowhere to rig a hairlight strobe... leastwise, to easily rig one.) Once again, I was looking out for reflections from my mainlight, still modified with the Octodome, and for anything else that might be reflected in the window behind her and that I didn't want seen in the images.
Bottom line when it comes to shooting in front of windows or other highly reflective surfaces, especially those that will produce specular reflections: Pay freakin' attention to details!
Christina captured with a Canon 5D, Canon 28-135 IS USM zoom, ISO 100, f/8 @ 160. 500ws monolight modified with a 5' Photoflex Ocotodome, slightly warmed with a few gold inserts, for main (and fill.) MUA Theresa.