Snapchat doesn’t make it obvious when someone deletes or blocks you. It’s one of the app’s many quirks.
Luckily, there is a way to know for sure if someone is no longer following you back. And it’s actually pretty easy to check.
If you're wondering whether someone follows you back, tap and hold on their name to see their Snapcode -- it's the yellow ghost icon that looks like this:
You can find someone's name by searching for them in the chat section of Snapchat or from your list of friends.
A person's Snapscore is the total sum of all snaps (photos and videos) they have sent and received in the app.