How do we know that Deinotherium has a trunk like other elephantine animals? The quick answer is that Deinotherium is a proboscid and therefore inferences into soft tissue anatomy that are missing will be based on extant animal models that are readily available. Those, in this instance, are the elephants that roam the African and Asian continents. Additionally, of course, soft tissue leaves visible attachment scars on the fossilized material that has been discovered around the globe. The tissue that attaches to the skull that indicates the presence of a trunk is unknown, to me, but is apparently evident enough that it does indeed lead to inferences for a trunk (probably in addition to the inferences that are made by the fact that the animal is related to extant organisms that tend to have trunks).