An indicator for enabling an analytical instrument to positively detect the presence of a vial in respective vial-receiving rack position regardless of whether the vial is oriented in a manner necessary for a code reader to read a code printed on the vial. Translation of a leaf spring upon insertion of a vial into a rack position causes a flag to obscure a code provided adjacent the rack position. An instrument into which the rack is installed interprets the failure of the code reader to detect this code as positive indication that a vial is disposed within the respective rack position.