Not knowing your existing structure it is hard to respond...
Where are you holding the Room occupancy info? I would assume you just added a Room field to the Sessions table.
I would also hazard a guess that Sessions have a start and finish time.
If all this is true, you could do a find for a the start and finish times of your required session and anything between, no records means its available.
I'm not sure if you are using a portal or a conditional value list for this but the underlying concept works the same way. You simply add more fields that compare the information on the room's availability as part of the relationship definition. Exactly how you do that depends on what fields you use to "book" the room for a given session.
If you post a little more information here, we can probably suggest a more detailed solution. Try listing each table involved and define the current relationships you use to link them.
It ended up being a problem with the way the relationships were set up. I found the information I needed in a posting on creating invoices that had a layout similar to what I needed.