Generally, I would suggest that you rather check this against the database.

But it depends on what are you going to do with the data - I assume that you need it on the client side (on the server side there is no reason not the check the DB) and that you want to use it to prevent event overlapping.

You can get a list of all events by checking dp.events array on the client side ("dp" being the ClientObjectName value). This works in both DayPilotCalendar and DayPilotScheduler. The array elements are actually "event parts". If there is an event that spans multiple days in DayPilotCalendar, it's shown as two boxes = event parts, one in each day.
You can check the following values:

• Start
• End
• PartStart
• PartEnd

And a few others (you will see the values if you open the HTML source of a page with DayPilot and scroll down to v.events = ""). The values are strings and can be converted to Date objects using new Date(dp.events[0].Start).

This is just a workaround:

• the .events array is not public and might change in the future (although it's not likely)
  
• it will involve quite a piece of code that will create the free time blocks from that information

However, I plan to add support for preventing event overlapping on the client side during move, resize and time range selection operations. Before it's available, you have two choices:

• play with the .events array
• check it in the server-side event handlers (you need to do this anyway) and use either CallBack + .Update("Not allowed") or PostBack + UpdatePanel + regular Label to pass the feedback back to the client

But you might as well be asking because of a completely different reason...