Translation:My brother will have returned from the war.
You can usually see it by the way the verb is written. For example vendt here comes from "at vende". vender - vendte - vendt. And as the word is in the past participle (the third word in the list) , that means it cannot be simply past. Then you see there is a vil in the sentence, but you already know the sentence cannot be present because the verb is in past participle. So vil cannot mean want in this sentence, so it can only mean will. So knowing you have a verb in past participle and vil as will means that you have some sort of future tense. Then you have the word være which means to have. So knowing you have will, have and a verb in past participle makes you know that it must be Future Perfect. Hope this helps!