If you have a large database of ancestors, you can easily overlook a relative who might appear in the 1940 census. Since there is so much information on this newest census, you will want to check for every person. I found a second cousin I did not have on a list. It turns out he was a young man of 18 years old living with an aunt and uncle learning a trade. The uncle I had in my database and when I located him in the 1940 census, there appears on the household listing this second cousin (his nephew), living with my aunt and uncle. Another mystery solved.
So how to run a check of all names that fit the 1940 time period, well if you have your databases in GEDCOM and on Ancestry.com you are in luck. Going to the Census & Voter List > 1940 US Federal Census on Ancestry.com and scroll down you should see the 'Hints' section listing for the family trees you have on Ancestry.com. It will give the number relating just to the 1940 US Census. It goes in alphabetical order (showing just the first couple people), with the person's full name and birth-death range. Just click on each one, the portion that says 'view all' to see the full list. In one of my family branches there were 53 persons on the 1940 census. So great because I could have overlooked some names for sure, plus just trying to find them in a location I hadn't expected is a time saver.
Another method to find this sorted into hints is to filter the collection so just the 1940 US Census hints for each individual is showing. So look for 'Collection Filter'.
If you have additional family branches you can do the same thing using the top tab titled 'Family Trees', then view the other family trees you already have on Ancestry.com. Filter the collection of hints to view those listed on a 1940 census.
This proved to be a very simple method to get everyone on my database. It is so easy to miss a person. Here the search is narrowed down for you.
A major drawback is if the transcription of the person's name (surname or given names) is way off. An example is an older half-brother of mine. His given names are Howard Francis. The transcriber had this 2 month old child's given name as 'James'. Now you think that those two names are not even simpler in any form. Luckily I had found him earlier in the 1940 census. On examining the handwriting by the census taker and faintness of the page, I can see how the transcriber could have mistakenly read James instead of Howard. So if you do find a mistake in a transcription, put the correction in 'Add Alternate Information'.
I highly recommend this method.