FileMaker only permits one user to edit a given record at a time. This is called "record locking" and a second user that attempts to edit a locked record gets an error message.
There is no built in function that can tell you how many users have a given record as their current record or in their current found set of records. You'd need to use script triggers to perform scripts to do this and any disconnects that force a user out of the database can leave the user as "viewing" a given record even though they are not in the database at all.
You could try placing a field in each table that can be used to test if the record is open by trying to write to it.
Write a script that will trigger OnRecordLoad
Set Error Capture [ON]
Set Field [ TestField ; "1" ]
If [ Get ( LastError ) > 0 ]
Go to Record/Request[Previous]
Show Custom Dialog ["Record in use"]
That is really neat, am going to try it, thanks much.
Without using a test field, you can use:
Set Error capture [on]
If [ Get ( LastError ) > 0]
Show Custom Dialog ["Record is in use..."]