You know you can use the visual icon(the eye of a person at the top of the screen). Now, we can put it into good use. First, build a bathroom. Now, open it and click the visual icon. Highlight the option reading "see-through rides" and if there are any people inside the bathroom, you will see them inside.

Note: I'm going to try and actually see the people up close in the bathroom, by selecting a guest who is heading toward a bathroom and watching them go in. Hmmmmm...