If I have a bool method, how can I see in the inspector whether it returns true or false?

You don't. The inspector can't show you what values methods are returning. If you want to see that, use the debugger. Using monodevelop, place a breakpoint (F9 on the line) where the method is being called, press F5 to connect the debugger to Unity. Run your game in unity with the play button. When the method gets called, the program will halt execution at your breakpoint, and you can step through the lines of code bit by bit with F10. You can hover over variables to see what they are. Press F5 again to continue the program as normal.

Mazer83's answers is also not correct. A standard view of the inspector can't display the value of a function. And to set the value of a public boolean would require some point of entry that updates whenever a change is made. During runtime this isn't a problem, however if you wish to display this value outside of runtime, while editing, then you'll have to deal with custom editors. These are scripts that allow you to manually draw the inspector window for a specific component type, they provide far more powerful functionality for editing, however are more complicated to design. https://docs.unity3d.com/Manual/editor-CustomEditors.html

Alternatively as that page also describes, you can add the [ExecuteInEditMode] to have the script get ran while not playing, depending on the complexity of the script this may or may not be a good option.

Although as for Mazer83's answer, while Debug.Log and setting the value during runtime will work fine for you in this case, it's still a very wise choice to get used to the debugger, because not all problems can be solved with simple logging, and even if a problem can be, often times the debugger is far, far more powerful. People wouldn't have designed debuggers if printing to console solved everything.