If it is not possible to set the JDEV_USER_HOME env variable (because Admin rights are not granted for the PC, for example), use a batch file to set the variable, and start JDeveloper from the command line:
Set either or both of the following profile options, at Site or User level:
ICX: Forms Launcher ?record=collect
Forms Runtime Parameters record=collect
Log out of the applications, and log back in again. When a core-Forms session is launched, you will see a message to say that Forms Runtime Diagnostics is enabled.
The trace file will be in $FORMS_TRACE_DIR on the applications server (not the database server). It will be named collect_PID.