When holding RMB, you can press Z to increase the FOV or C to decrease it. Keep in mind though, that the moment RMB is released the FOV will reset to the value in the scene settings.
Daily Unreal Column #18 - Temporary FOV preview
If you ever look at your level and though of quickly zooming in or out by temporarily tweaking FOV (Field Of View) value, there is an easy way to do it.
