Transdermal insertion of a transcutaneous filling apparatus, for the purpose of filling a fill chamber of an implanted therapy delivery device, is monitored by measuring each impedance between pairs of electrodes of a needle of the apparatus, and comparing each to a threshold impedance; the electrodes, preferably at least three in number, are isolated and spaced apart from one another along a length of the needle. A confirmation signal is generated when at least one of the measured impedances is greater than the threshold impedance, and another is less than the threshold, the condition indicating that one of the electrodes is located within a non-conductive septum, through which the apparatus must pass to access the fill chamber. A detection circuit, which may be located in a housing of the apparatus that is attached to a proximal end of the needle, measures and compares the impedances.