I not sure about the forms with trace enabled. I don't think so it is possible to enable trace at forms level and replicate it across all instances of forms. Trace is always enabled on database session level. You can use via forms,sql and other methods.
You can use below 3 columns of v$session/gv$session to identify whether trace is enabled or not for that SID and then you can work backwards to find out FORM_NAME/application user name.
SQL_TRACE-----------VARCHAR2(8) Indicates whether SQL tracing is enabled (ENABLED) or disabled (DISABLED)
SQL_TRACE_WAITS---- VARCHAR2(5) Indicates whether wait tracing is enabled (TRUE) or not (FALSE)
SQL_TRACE_BINDS ----VARCHAR2(5) Indicates whether bind tracing is enabled (TRUE) or not (FALSE)