How to … Tell if a specific epithet is preoccupied
A specific epithet is preoccupied if the same combination of a generic name and a specific epithet already exists in a validly published species name, even if it is not in current use. If you validly publish a new species name with the same genus-plus-epithet as a name that has already been validly published (with a different type), your new name will be a later homonym and therefore illegitimate.
The rule on homonymy (Art. 53) applies within the Code for algae, fungi, and plants: that is, you may not name a plant using a name that has been previously validly published for an alga or a fungus, and vice versa. Note also that names of fossil algae, fungi, and plants are also covered by the Code, so if a new name for an extant organism has been previously published for a fossil organism, the new name will be illegitimate. It is therefore important to check algal, fungal, and plant names of both extant and fossil organisms when deciding whether a name is available.
Note that preoccupation does not extend to groups governed by other nomenclatural Codes (Art. 54.1). That is, a new algal or plant name can be legitimately published with a combination that has been previously used for an animal or a prokaryote (though this should be avoided whenever possible). However, when a new fungal name is published on or after 1 January 2019, the combination must not have been previously used for a protozoan or prokaryotic or taxon, or else it will be illegitimate (Art. F.6.1).
There is currently no comprehensive, online registry or index of fossil names. If in doubt, consult a palaebotanist.
For more information, see Chapter 5 How to publish a new name in The Code Decoded.