You have to just use controller_action_predispatch_checkout_index_index action to check before checkout index page is load.
<observer name="checkcout_page" instance="Custom\Module\Observer\CheckShoppingCartObserver" />
If you want to fire event before any action is called you have to use controller_action_predispatch_XX_XX_XX
If you want to fire event after any action is called you have to use action event controller_action_postdispatch_XX_XX_XX
In above section XX_XX_XX is handlename/action of page.
You can call any action in pre and post dispatch event for each page.
In your case checkout_index_index handler is used.
Just for example,
If you want to do something before checkout cart page is load you have to use event controller_action_predispatch_checkout_cart_index
If you want to do something after checkout cart page is loaded, you need to define controller_action_postdispatch_checkout_cart_index action and set your logic in observer file.