How did Steve find out..
that Arthur killed his dad?
shareWhen loading up gear "from the top shelf" he found his father's prized, custom engraved, gun.
shareWhen Steve was pulling all the bags from the top rack to pack the boat, one fell and he found his Father's gun in Arthur's stuff.
Of course, that doesn't really mean anything. His Father could have given Arthur the gun. But without assuming and jumping to conclusions, you wouldn't have a movie.