Frequently you may have trouble tracking down death dates of ancestors, especially if the died before localities were required to officially keep vital records.
Here are a few suggestions on where to look for or how to determine death dates:
(1) Death certificates and records - easiest first if you can locate them.
(2) Cemetery records: burial permits, day books, gravestones. You may get a range in information from exact date to year only.
(3) Newspaper obits, death notices, funeral notices.
(4) Probate: Note the date of the will, if there is one, and the date of the probate or the date the inventory was taken. The death date will probably be close to the probate. It may be a few days or a couple of months.
(5) Marriage records: If there is a 2nd marriage [or 3rd, etc.] for an ancestor, that can give you a clue to the date of death of the spouse. If a man or woman had young children when the spouse died, he/she probably remarried within a year or two. At least you have a ballpark figure to work with. Beware of divorces! The departed spouse may have just left, not died!
(6) Births: The birth of a child can help in determining death dates. Look at the birth of the youngest child, if the father remarried. Chances are his wife died in childbirth or shortly thereafter. [You need that remarriage for this to be a good option.] Also note that in some cases the child may have been born after the father died, so beware of that.
(7) Census records: It may be a 10 year gap, but it's a starting point. Just make sure you check to see if the ancestor moved or the name was misspelled next time. Also check to see if he/she is in a later census.
(8) Taxes: Some states keep wonderful tax records. Follow your ancestor until he doesn't appear again. Then check ahead a year or two just to be safe. If you've reached a dead end, your ancestor has probably died or moved.
(9) Local histories & records: Search for the ancestor to see if he/she appears. A death date or, at least, a time frame might be given.