They are very open-ended questions. You did not state what device people are "watching" or give any information with regards to how your device is setup and how one would view said device.
If we assume a single IP camera which is connected to a NVR or VMS PC, then break it down.
Are the cameras isolated on their own network segment or do they share the network with other devices?
Is the only way to view camera footage via the NVR/VMS? Is a strong password requried for this? Who knows this password? Do you have seperate accounts for individual users or do you share passwords? When was the last time you audited your accounts? Could Ex-employees still have access?
Who has physical access to the NVR/VMS to view camera footage?
Can the NVR/VMS be accessed remotely? If so, how is this configured and who
should have access?
AS far as telling if anyone is accessing various devices, all my Dahua cameras have a local log which will give you logon/logoff/configuration saving events etc.
I would assume that most NVR's would have the same. If running a VMS such as
BlueIris then obviously you have the Windows Event viewer to see which account logged onto the device and then within BlueIris there are logs relating to opening the local console and remove viewing via the web service (details contained in this are good as they provide Ip address of connected machine, hostname of connected machine and username used to authenticate with BlueIris. - Of course, if you have just set everything up with Username/password which everyone uses then this is of limited value as you cannot tell who has logged on. But it would give you timestamps.
What makes you think someone is accessing your devices? Without trying to get too Hollywood, could someone be viewing a device of their own which has been left in your business?