You can't really know if someone blocked you on Snapchat, but there a few signs you can guide on. Firstly, you won't see that person online and all the messages you send won't appear as viewed. Secondly, even if you add them back, the request will stay as "Waiting confirmation" for a very long time. This could also mean they've deleted their account as well. The best way is to ask him/her through other means.