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.
This course is really inconsistent with future tense, some questions like this one will mark you wrong for not using it and others will mark you wrong if you do use it.
It so depends on context. Rather than past present future, Japan uses past or not, with variations for what kind of "not past" we're talking about
It has to be future because "this evening" is something that is going to happen. Japanese doesn't have a dedicated future tense.
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.
But there is a different tense for currently falling. That would be 今ばんゆきがふっているです。for if it's currently evening and snowing concurrently. So we know the sample sentence will be in the future.
There is only past and non-past. Other information indicates it's in the "future".
I would go with よるはゆきがふります. The tense is the same but it makes no reference to specific times so it's likely to be inferred that you're speaking in a general sense.
How can you tell the tense? Surely it all depends on context and "it is snowing this evening" should be an acceptable answer.
今晩：こんばん（今日の晩）＝this evening or this night
夕べ；ゆうべ（きのうの晩）＝last evening or last night
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。?
in past tense, it would be "furimashita" i think, but present tense is the same as future "furimasu"