try {
	get-label "Stopped"
	// If no label with the text "Stopped" exist then an exception
	// will get thrown, the next statement will not get executed,
	// and the script will jump to the -catch block of the
	// surrounding try
	get-button "Start in standard mode" | click
} -catch {
	try {
		get-label "Idle"
		get-button "Start in XXX mode" | click
	} -catch {
		assert-true false -message "No matching label found."
	}
}

proc buttonExists [val Name] {
	try {
		verify-error {
			get-button $Name
		}
		bool false
	} -catch {
		bool true
	}
}

//now you use it in your test code like:
if [buttonExists "My Schroedingerian Button"] {get-button "My Schroedingerian Button" | click}