You can check it using javascript below method,

if(!window.checkoutConfig.quoteData.customer_id){
  var getGuestEmail = window.checkoutConfig.quoteData.customer_email;
  alert(getGuestEmail);
}

Using above code you can get guest email id. I haven't check with php methods.

You can get it using javascript in checkout page using below method,

if(!window.checkoutConfig.quoteData.customer_id){
  var getGuestEmail = window.checkoutConfig.quoteData.customer_email;
  alert(getGuestEmail);
}

Using above code you can get guest email id. I haven't check with php methods.