I think for a lot of aces, at least before "asexual" became part of the common orientation lexicon, it's a journey of tiny steps, and more a process of elimination of other orientiations rather than a sudden realization. Perhaps the younger generations will be able to know right away due to it being more common in everyday language/visible.
I went through quite a rolodex of orientations. I didn't feel 'straight", I knew something was..."off".