You can find this out by accessing the businessObject of an element
var elementRegistry = modeler.get('elementRegistry'),
task = elementRegistry.get('Task_1ycg1p3'),
businessObject = task.businessObject;
var outgoingConnections = businessObject.outgoing;
var incoming = businessObject.incoming;
Which will get you the connections for the task. With them, you could then indirectly get the connected elements with the outgoing and incoming properties once again.