I know a plan is good when my bottom-up estimates (padded with a buffer for unforeseen contingencies) align with my top-down gut instinct of how long a project of a certain complexity "should take".

As an experienced professional, one often compares with past projects and looks at the duration of a similar one - not necessarily regarding content but complexity.

(I also conduct research on methodology, e.g. https://arxiv.org/abs/2201.07725 / https://dl.acm.org/doi/abs/10.1007/978-3-031-08473-7_48 .)