This is computationally intensive (more so than may be evident at your first thought) and there are many different possible desired semantics of such, and hence quartz does not provide a high-level api for doing this.
Your will basically need to iterate over every trigger in the scheduler, and determine whether it will fire within your time range. There are some helper methods on the TriggerUtils class that will compute future fire times of a trigger (don't just look at the trigger's nextFireTime - because the trigger may repeat a number of times... one or more of which may occur within your time range of interest, or perhaps the repeat interval may cause the trigger's firings to completely skip over your time range of interest).