In this case, the nice curved edges of the aliens spacesuits are the "reflected window" giveaway.

This mostly seems to happen with replacement storm windows, which can have thin sheets of glass and be cheaply manufactured. The glass ends up slightly concave (or convex), creating a lens with square edges.

If you know the approximate size of the aliens curves (looks like about 10cm to me) and the distance to the window (maybe 20-30 meters?), you can get a rough approximation of how badly the window is distorted by just taking the ratio. With those numbers, the deflection is only few millimeters.