This is the discussion page for users only. If you want to contact Duolinguo's employees, you need to click (in the app) on the little flag under the exercise. I really wonder why it's only in the Hindi course that there are users who post developer feedback in the user forum
It is something you come to learn. You can get a hint from the ending of its adjective, or (if it is the subject) of the ending of the main verb.
Also the noun ending can give a hint: -ा (ā) endings tend to be masculine, -ी (ī) endings tend to be feminine. Focus on learning the exceptions, e.g. आदमी - man, पानी - water and पक्षी - bird are masculine; माता - mother is feminine.
(Past tense verbs, which we haven't yet reached, have more complications. So best learn the gender as soon as possible when learning a new noun.)