Depends on if all threads were put into a paused state. Also, the program that's being debugged might have spawned some additional processes that will be checking if the parent is being ptraced.

Replace "self" with the PID you want to check if it is being traced:

  $ cat /proc/self/status  | grep Tracer                                                                                                                                                                                                                                                                                                                                                                       
  TracerPid: 0