Question: how do you know if someone unfriended or blocked you on snapchat?
Answer: If you're wondering whether someone has unfriended you, tap and hold on their name to see their Snapcode — the yellow ghost icon that looks like this: ... 
If you mutually follow each other, you'll see their Snapscore, which represents the total sum of all snaps (photos and videos) they've sent and received in the app.