Context, mostly. "Sont" is a verb and "son" is a possessive adjective. Also, "sont" is plural and "son" is singular. So you can find clues within the sentence. (Not the easiest thing to do, I know.)
If "sont" is followed by a word that begins with a vowel, the speaker will sometimes pronounce the 't' at the end of "sont," thus: "they are happy" == "ils sont hereux" could be pronounced with a hard 't' sound before "hereux"
" His" in English can be both "Son" or "Sa." it completely depends on the gender of the object, not the subject. same with "Her." Thus: "son sac" can be either his or her bag. in the same way, "sa valise" can be either his or her suitcase.
it's not possible to know if it is a man or woman from the scanty information provided in this exercise.
Without context, Orange, we do not know. I bodes well to remember that this course is not a holiday phrase book but a language learning course where our focus will be on grammar subject/object (and Sac Bag is the object noun and is masculine) noun gender (which more often than not needs be memorised and not assumed nor confused with the subject noun), sentence structure, verb conjugation, singular/plurals. In other words, language, not "sense." On this course, we are a whale, a fly, an ant, turtles eat pasta, we drink a red fluid and we don't know whether it is red wine, fruit juice coloured red or blood. We learn that the main clue as to the gender of the object noun (but not the "sense" of the phrase/sentence) is in the article (although with plurals that is not clear, which is why noun gender needs be memorised. For example the vagina is a masculine noun even though, when born, males do not have a vagina, only females do. LE Vagin Males are born with a penis and strangely enough that object noun is also masculine gender.) Are you beginning to understand from whence a language learning course comes, now? Lastly Duolingo does indeed have its little flaws but it is free and one may spend over £500 at Babbel, Rosetta Stone and other courses which charge and yet come across little flaws. Incidentally, on a language learning course, there is an apostrophe in "Don't" as I have shown. Avec respect et bonne chance, JJ.