Monolayers of hexagonal boron nitride (hBN) have been – besides their high relevance in 2D materials research – traditionally been very difficult to handle due their lack of optical resonances making them essentially invisible in any optical microscope. Making use of the strong infrared resonance associated with a lattice vibration in hBN, the recently developed sum-frequency generation (SFG) microscope was shown to drastically enhance imaging contrast, enabling live-imaging of hBN monolayers. Furthermore, the phase-resolved SFG signal enables absolute determination of the crystal orientation which even allowed to extract the atomistic edge termination of triangular hBN flakes.