  The interplay between crystal nucleation and the structure of the metastable
fluid has been a topic of significant debate over recent years. In particular,
it has been suggested that even in simple model systems such as hard or charged
colloids, crystal nucleation might be foreshadowed by significant fluctuations
in local structure around the location where the first nucleus arises. We
investigate this using computer simulations of spontaneous nucleation events in
both hard and charged colloidal particles. To detect local structural
variations, we use both standard and unsupervised machine learning methods
capable of finding hidden structures in the metastable fluid phase. We track
numerous nucleation events for the face-centered cubic and body-centered cubic
crystal on a local level, and demonstrate that all signs of crystallinity
emerge simultaneously from the very start of the nucleation process. We thus
conclude that there is no precursor for the nucleation of charged colloids.
