  U(1) gauge symmetries in F-theory are expected to manifest themselves as
codimension three singularities of Calabi-Yau fourfolds. However, some of these
are known to become massive at strong coupling via the St\"uckelberg mechanism.
In this note, we propose a geometric picture for detecting all U(1)'s, and
determining which ones are massive and which ones are massless. We find that
massive gauge symmetries show up as codimension three singularities that only
admit small, non-K\"ahler, resolutions. Our proposal passes several highly
non-trivial tests, including a case with a non-diagonal mass matrix.
