how to get the states of other objects in model plugin

I am learning how to write a model plugin to control an object and interact with other objects in Gazebo. In the [model plugin tutorial](http://gazebosim.org/tutorials?tut=plugins_model), I see we can get update from "void OnUpdate(const common::UpdateInfo & /*_info*/)". However, the updateInfo does not seem to contain the poses and motion characteristics of other objects. Could anyone point me to any sample code to do that? Also, how do I know if my object has a collision with other objects? Are there any events to listen to? btw: I'm using the direct Gazebo plugins, not the ROS interface yet. Thank you!