Permission from the publisher
If exclusive use rights were transferred, this was usually done in a publishing contract. In this agreement, authors may also have been granted explicit permission for self-archiving. If this is the case, authors can apply this accordingly.
In addition, publishers often have policies for self-archiving in which they specify the requirements and conditions for this. Such policies are particularly common for journals. These policies often use the actual term self-archiving, but can also be found under terms such as repository policy, reuse policy or sharing policy, or as part of the publication guidelines for authors. A valuable tool is the Sherpa Romeo database, which aggregates and summarises the policies of many publishers and journals and provides links to the publishers' websites (where the policies should be double-checked as a precaution).
If there is neither permission in the publication contract nor a policy for self-archiving, a request to the publisher can be made. Such requests are quite promising, as the publishers in question are often quite pragmatic in granting individual authorisations for self-archiving. Sometimes publishers even provide the full text at the same time.