First, rule out the typing
Most dead identifiers are transcription errors. A digit dropped, a character transposed, a trailing full stop from the end of a sentence swept into the string. Copy the identifier from the source record itself rather than retyping it, and try again.
Line breaks are the other frequent culprit, because a long identifier wrapped across two lines in a document often picks up a hyphen or loses a character when copied back out. This accounts for a surprising share of the panic.
Then check whether the record moved
Journals change publisher, merge and occasionally close, and while identifiers are meant to survive that, the redirection sometimes lapses. Searching the exact article title will usually find the record living somewhere else.
If the article turns up under a different publisher with a different identifier, use the one that currently resolves and note it. The point of the field is to be a working route to the source, and an identifier nobody can follow is not serving that purpose.
Consider retraction and preprint status
A retracted paper generally keeps its identifier and gains a notice, so a resolving link with a retraction banner is a different situation from a dead one — that source can sometimes still be cited, explicitly as retracted, if retraction is what you are discussing.
Preprints have their own identifiers and are not the published version. Where a preprint has since been published, cite the published article, because the two can differ substantially after review.
Then treat it as possibly invented
If the identifier does not resolve, the exact title returns nothing, and the authors have no publication matching it, you are most likely looking at a fabricated reference. These arrive through generated bibliographies and they are formatted perfectly, which is precisely what makes them dangerous.
Remove it and find something real. Do not repair it by attaching the identifier of a different paper to the same entry, which produces a citation that resolves to a source saying something else entirely — a worse failure than the one you started with.
What to do with what you learn
A dead identifier that turns out to be a typing error tells you to copy identifiers rather than retype them. One that turns out to be a moved record tells you to check the publisher first next time. One that turns out to be invented tells you something about where that reference came from.
That last case is worth pausing on. If one entry in a list is fabricated, check every other entry rather than assuming it was isolated, because invented references arrive in groups far more often than they arrive singly.
The habit worth building
Check identifiers as you collect sources rather than at the end. A dead link found while you are still reading costs nothing, because you simply find something else. The same dead link found the night before submission costs you a paragraph you have already built an argument around.
That is the general shape of source work: every check is cheap early and expensive late, and the entire difference is whether you are still choosing sources or already depending on them.
Questions this raises.
How long should this check take?
Under a minute per reference once you have done it a few times: paste the identifier, see if it resolves, and if not search the exact title. On a list of twenty-five references that is under half an hour, which is considerably less than the time spent worrying about whether a list is sound.
Do all sources have identifiers?
No, and their absence is not a problem. Books, reports, guidelines and older articles frequently have none, and your citation style has a form for that. Where one does exist, including it is expected in most styles because it is the shortest route your reader has to the source.
Can I cite a retracted paper?
Sometimes, and only deliberately. If your topic concerns the retraction, or you are documenting how a claim propagated, citing it explicitly as retracted is legitimate and useful. Citing it as evidence for the claim it originally made is not, and markers in most fields will recognise well-known retractions on sight.
Should I include identifiers if my style does not require them?
Where one exists, include it unless your style explicitly forbids it. It is the shortest possible route from your reference list to the source itself, which is the entire purpose of a reference list. Leaving it out makes checking your work harder for anybody who wants to, including you in six months.
What if my source genuinely has no identifier?
That is completely normal for books, reports, guidelines and older articles, and every citation style has a form for it. Use a stable URL where one exists and an access date where your style requires it. The absence of an identifier is not a mark against a source, and an identifier that does not work is.
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