As far as I know, the only way to check that would be to check the "User Detail" of your Notes mail DB (which actually contains both your e-mail and your calendar). Since other people should only have access to your calendar (e.g., not your e-mail messages), any user activity of others listed for your mail DB should indeed be them checking your calendar.
To check this:
1. Go to any folder/view of your mail DB (i.e., your "Inbox" or your calendar)
2. Bring up that DB's "Properties" window (i.e., by clicking on File --> Database --> Properties)
3. Go to the second tab of the "Properties" window (i.e., the "Info" tab which has an "i" on it)
4. Click on the "User Detail" button to make the "User Activity" window appear
5. OPTIONAL: You may want to sort the "User Activity" window's info by the "User" column (i.e., this is especially helpful if your mail server is in a "cluster" or if your company has a BlackBerry Enterprise Server which is frequently checking your mail DB)
6. Scroll up/down in the "User Activity" window, as needed, to see who all has been accessing your mail DB
NOTE: The tracking of user access (in the "User Detail" information) can be enabled/disabled ... via the checkmark boxes in the bottom-left corner of that same "User Detail" window. If it is currently disabled on your mail DB, you can (probably) enable it ... but you'll then need to wait to find out who all is accessing your mail DB (e.g., calendar) - there's no way for you to determine who already has been accessing it (while your database wasn't keeping track).