We had the same issue this year, and unfortunately there's no easy way to check your progress throughout the year. I discussed this with a customer representative when I went to get my refund, and she said that they are going to change their interface in the next few years to make it easier. It's frustrating, I agree. In some respects, it would be better if they just gave the percentage discount upfront on eligible items. This is what Target does with the Redcard. Then, if you didn't reach your extra executive membership fee, they could just mail you a check in the mail for the difference.
Edited to add that I love Costco so much I've never thought to downgrade my membership now that I have the executive one, especially since they refund the cost back. Who knows if the next year we will buy a large ticket item that will push us over the membership amount?