This page gives the following examples of correct punctuation showing possession for singular nouns ending in -s:
- Thomas's job
- the bus's arrival
- James's fiancée
- Steve Davis's victory
VS
- Socrates' philosophy
- Saint Saens' music
- Ulysses' companions
- Aristophanes' plays
The page says: "a name ending in s takes only an apostrophe if the possessive form is not pronounced with an extra s."
As a learner of English, how do I know whether a particular name (say, Alexis or Andres) should be written (and said) with the extra s?