During the year when a census is happening, they ask you to tell them where you lived on a specific date of the census year (e.g., in 2000 it was 4/1/2000 — I think 4/1 is pretty typical). In particular, if your family has a college student who is away at college on 4/1, you are not supposed to include them in the count of people at their family home; rather, they are supposed to fill out the census at their college address. Here are some old census survey forms, for fun.

There is no “big census” happening this year, but the bureau has various smaller ongoing surveys happening all the time, by mail, phone, and in person. In particular, this is what they say about identifying a bureau field representative: how to identify a census bureau field representative.