Go to this page. Wait for the page to finish loading completely, scroll down, and enter your zip code. This will tell you if you live in an area that is eligible for free same-day delivery.
If you add at least $35 of eligible products to your cart, you should see same-day delivery as a shipping option. Also, eligible items will display "Prime FREE Same-Day" or “Prime FREE One-Day” on their detail page. If you don't see this, it doesn't apply!