I've published a paper in viXra and I would like to know if someone else cited it anywhere, but I don't know how. Is there a kind of global index for citations? Does it include “alternative archives” such as viXra?
Most established academic journals are indexed in what are logically called “journal indexes”. These indexes commonly record all publication metadata, and usually also include a record of all citations. Such indexes are thus called citation indexes.
Now, the problem with viXra is inherent to its “alternative” nature: it is most definitely not indexed by conventional players (Web of Science, CrossRef, PubMed, CAS, etc.). You thus have two ways of looking for citations to your paper:
- Google Scholar includes content from viXra in its results (see this search’s first hit, for example). It probably also records citations of such articles, but the quality of citation matching is limited by Google's heuristic algorithms. They're good, but not as good as DOI-based mechanism that commercial databases nowadays employ.
A regular web search with you name, or the title of your paper. This is not as automated, but it may give good results as to who is citing your work, especially if this is one of your first papers and you don't yet have a large web presence.
(Obvious caveat: if your name is John Smith or Li Wang, web search will not help much.)
Vixra is indexed in Microsoft Academic Search, as of Aug 20, 2019. While potentially not as useful as Google Scholar, it does provide a similar portal; of course, Google itself also indexes Vixra papers (but not Google Scholar, which only apparently indexes the citations.)
Try using Google Scholar. There may be field-specific indexes too.