Bureau of the Census Catalog and Guide
From 1790-1972, this is a straight pdf of the print catalog. Page through to the relevant census year (e.g. 1910 is the Thirteenth Census). Read through the descriptions of what was published in each section.
--Was the data gathered in the census for the year in question? (Was the question asked?)
--Was the data published for very small towns, only for towns over a certain population, or only at the county, or state level?
--Within the descriptions, the guide will mention the lowest geographic level represented (e.g. 1,000 or more inhabitants, 10,000 to 25,000, etc.)
When the catalog doesn't specifically identify the geographic level covered (e.g. when it doesn't say something like "for states, counties, minor civil decisions with X number of inhabitants"), it is describing a report that has more general, summary-level national statistics.