How can I determine that Windows Installer is performing an upgrade rather than a first time install?

<!-- Property definitions -->
  <?define SkuName = "MyCoolApp"?>
  <?define ProductName="My Cool Application"?>
  <?define Manufacturer="Acme Inc."?>
  <?define Copyright="Copyright © Acme Inc. All rights reserved."?>
  <?define ProductVersion="1.1.0.0"?>
  <?define RTMProductVersion="1.0.0.0" ?>
  <?define UpgradeCode="{EF9D543D-9BDA-47F9-A6B4-D1845A2EBD49}"?>
  <?define ProductCode="{27EA5747-9CE3-3F83-96C3-B2F5212CD1A6}"?>
  <?define Language="1033"?>
  <?define CodePage="1252"?>
  <?define InstallerVersion="200"?>

<Upgrade Id="$(var.UpgradeCode)">
      <UpgradeVersion Minimum="$(var.ProductVersion)"
    		  IncludeMinimum="no"
    		  OnlyDetect="yes"
    		  Language="$(var.Language)"
    		  Property="NEWPRODUCTFOUND" />

      <UpgradeVersion Minimum="$(var.RTMProductVersion)"
    		  IncludeMinimum="yes"
    		  Maximum="$(var.ProductVersion)"
    		  IgnoreRemoveFailure="no"
    		  IncludeMaximum="no"
    		  Language="$(var.Language)"
    		  Property="OLDIEFOUND" />

</Upgrade>

<!-- Define custom actions -->
<CustomAction   Id="ActivateProduct"
    		Directory='MyCoolAppFolder'
    		ExeCommand='"[MyCoolAppFolder]activateme.exe"'
    		Return='asyncNoWait'
		Execute='deferred'/>

<CustomAction   Id="NoUpgrade4U"
    		Error="A newer version of MyCoolApp is already installed."/>

<InstallExecuteSequence>
    <Custom	Action="NoUpgrade4U"
		After="FindRelatedProducts">NEWPRODUCTFOUND</Custom>
    <Custom	Action="ActivateProduct"
		OnExit='success'>NOT OLDIEFOUND</Custom>
</InstallExecuteSequence>