The scripting terminology for the Keynote application class is spread across three suites in the Keynote dictionary:

The Standard Suite Application Class

The Keynote Suite Application Class

The Keynote Compatibility Suite application class contains a set of commands for controlling the delivery of presentation documents:

The Keynote Compatibility Suite Application Class

Intrinsic and Hidden Properties

In addition to the three suites shown above, Keynote has intrinsic application properties shared by all applications. These properties can provide the application identifier and the activity state of an application (is the application open).

Also, as a very useful bonus, the Keynote dictionary contains a set of hidden properties, including one for indicating whether Keynote is presenting a document, and whether the slide switcher is visible or not:

Intrinsic Application Properties and Hidden Keynote Properties
Open in Script Editor
01		-- INTRINSIC PROPERTIES FOR ALL APPLICATIONS
02		get the id of application "Keynote"
03		--> "com.apple.iWork.Keynote"
04		get running of application "Keynote"
05		--> true
06		get the version of application "Keynote"
07		--> "6.2"
08
09		-- HIDDEN KEYNOTE APPLICATION PROPERTIES
10		tell application "Keynote"
11		get properties
12		-- RETURNS: {frozen:true, class:application, playing:false, version:"6.2", slide switcher visible:false, name:"Keynote", frontmost:false}
13		end tell

Starting and Stopping Presentations

Using elements from the sources and methods shown above, the following script contains a statement for Keynote to stop presenting. This statement will only run if: 1) Keynote is currently launched and running; and 2) a presentation is currently playing:

Stop Presentation
Open in Script Editor
01		-- IF KEYNOTE IS ACTIVE AND PLAYING THEN STOP IT
02		if running of application "Keynote" is true then
03		tell application "Keynote"
04		if playing is true then stop the front document
05		end tell
06		end if

And a script for presenting the front document. It will start the presentation only if Keynote is open with a document open:

Start Presentation
Open in Script Editor
01		-- IF KEYNOTE IS ACTIVE AND NOT PLAYING THEN START IT
02		if running of application "Keynote" is true then
03		tell application "Keynote"
04		activate
05		try
06		if playing is false then start the front document
07		end try
08		end tell
09		end if