"Manges-tu du poisson ?"
Translation:Are you eating fish?
In most cases you don't have to differentiate. Continuous tenses are not as common as in English. Also, often you can get the mode of continuity from the context. But when it's important to tell, then instead “Je mange du poisson” you can say “Je suis en train de manger du poisson” (literally like: I am in the process of eating fish).
New grammatical phenomena are introduced in the tips & notes section of the respective skills. You find them by clicking on the light-bulb button of the skill – in a web browser. Unfortunately this feature is not available in the mobile app.
One method of making a question is to take the corresponding statement and invert subject and verb.
Find the tips & notes of the skill “Questions” here: https://www.duolingo.com/skill/fr/Questions/tips-and-notes