Until today, iron gall ink is classified as an exceptional underdrawing material for paintings. A certain identification is always based on invasive analysis. This article presents a new non-destructive analysis approach using micro-X-ray fluorescence scanning (MA-XRF), LED-excited IRR (LEDE-IRR) using a narrow wavelength-range of infrared radiation (IR) and stereomicroscopy for visualising and identifying iron gall ink underdrawings. To assess possibilities and limits of this non-invasive approach, results were compared to invasive examinations on cross-sections using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX). The approach is tested on panel paintings of Hans Holbein the Elder and Giovanni Battista Cima da Conegliano. The holistic setup could successfully visualise underdrawing lines made with iron gall inks, which formerly remained invisible by means of conventional IRR. For the first time, a direct access to a formerly invisible type of underdrawing is created, allowing to harness the whole iron gall ink underdrawing for interdisciplinary studies.