Translation:It will snow this evening.
By context, and also by tense. If you're not in the evening and say, "this evening, [snowfall]", then it's pretty clear it's in the future. If you are in the evening, then you would rather use the continuous "snow is falling" ゆきがふっています.
All in all, I would find it more natural to say ゆきがふるでしょう though, where でしょう expresses a guess that something will happen.
How do you know it is something that is going to happen? You would still say "this evening" in English if it was already the evening. Like "it is cool this evening", or "Snow is falling this evening", which looks like the literal translation of the Japanese to me. Maybe it is a bit unnatural but I don't see a problem with it.
Duolingo doesn't give us any context for this things. Putting it in the future is an assumption and other assumptions are just as valid. If it the snow has stopped falling but it is still evening I might well say "It snowed this evening" or perhaps "Snow fell this evening." If I wanted to say that in Japanese what would I say different than こんばん、ゆきがふります。?