How do I pull data from aSharePoint calendar?

We have several conference rooms in our facility and we are using SP calendars for the scheduling tool.  I am needing to build some sort of conflict alert workflow for users so that when they schedule a room on top of another previous appointment I can send them an email letting them know that there is, in fact, a previous appointment and that they need to go in and change theirs.  What I have in mind is for the user to enter their appointment, then have a workflow that checks the calendar for an appointment on that day, that room, that time, etc., and compares the two to make sure that they don't conflict.  Incidentally, the calendars are all views from the same document library.  All appointments go to the same library and are separated by room for the calendar views.  As previously stated, I need to be able to pull the appointment data and compare inside a workflow that I will build using SP Designer.  Any help would be appreciated.  Thanks!