Most of the time you can find the information you want without actually dealing with the class internals and global variables. There are a whole bunch of functions that you can call from anywhere that will enable you to get the information you need.
There are two main scenarios you might want to use WP_Query in. The first is to find out what type of request WordPress is currently dealing with. The $is_* properties are designed to hold this information: use the Conditional Tags to interact here. This is the more common scenario to plugin writers (the second normally applies to theme writers).