As far as I know there is no simple built in way to know who else is using the table. You can always check the server console but this is not very convenient in many situations.
We use our in-house developed utility 'WhoHasWhat' which tells us which users are using any file (including dbf tables etc). This utility is based on GPLIBw and has some restrictions: it only works for Novell server, the user must have console operator rights, it only works in DOS. With the utility we can also in emergencies disconnect a user remotely (usually used when the user has gone home for the evening but left a table in use!)

The usual work around if you have this situation is to use semaphore locking i.e. the opening of tables by users is registered in a table which amongst other functions can, at a glance, tell you who has what table open. I don't like this as a general solution because you have to be confident that all developers and any interactive users only open tables via the semaphore - something which is hard to enforce.

Maybe someone else has a more modern, more efficient utility.