This article provides an example of how to use the Client API to GET the latest version of an item by xID. 

var lowVersionItem = broker.GetItem(SWHandleUtility.ToHandle("x04000000000A01E1"));
IswItem lastVersion = lowVersionItem;
while (lastVersion.NextTrunkVersion != null)
{
    lastVersion = lastVersion.NextTrunkVersion;
}

For example, given the xID x04000000000A01E1 in the above example, this would return the item with xID x04000000000A84A7 which is Version 2. Version 2 in this example is the latest version of the item.