...
You can always check the current zoom percentage via the zoom indicator, located, by default, at the lower right corner of the screen. 
Using a Wheel Mouse
...
Hotkey: <+> or <-> (on numeric keypad) 
Magnifies or shrinks the current view. The zoom factor is equal to two by default, but you can change this in the Advanced Preferences page of the Program Setup.
...
Zoom Window
Hotkey: Ctrl+Home 
Zooms to an area defined by a rectangle. The rectangle is defined by selecting its two opposite corners, or by dragging the mouse from one corner to the other.
...
Zoom Extents
Hotkey: Ctrl+Backspace 
Zooms to an area that completely contains all visible objects (including lights and camera objects). This is the same area enclosed by the selection rectangle if you select all the objects.
Any objects on an invisible layer will not be taken into consideration.
Zoom Full View
Hotkey: Shift+Backspace 
Only available in Paper Space, displays the entire drawing sheet. The drawing sheet may consist of multiple printer paper sheets, indicated by dashed lines.
Zoom Selection
Hotkey: Alt+Backspace 
Zooms to an area that completely contains all currently selected objects.
...
Tip: You can also use the Design Director to quickly zoom on an object or group of objects. For a single object, see Design Director- Graphics. For a group of objects, see Design Director- Categories.
...
Printed Size
Zooms at approximately the size the drawing will be when printed. In this view, one inch of screen space is approximately equal to one inch on paper. The exact ratio of screen space to Paper Space will vary, depending on your graphics display and screen resolution.
...