EDIT: Now that we are publishing ConstraintLayout directly on maven, you can check the maven directory:
https://dl.google.com/dl/android/maven2/com/android/support/constraint/group-index.xml
Look in the SDK Manager in Android Studio. If you check the “Show Package Details” in the SDK Tools section, you should see all the available verions of ConstraintLayout (under Support Repository -> ConstraintLayout for Android)