Check their Snap Score; if it appears, they added you back
Try sending them a Snap; if it says Delivered, they are likely added back
Look for the Chat option; if you can chat normally, they added you back
Check if you can view their private Story, if they share one with friends
See whether their name appears in your Friends list instead of only in Added Me
If you can call them on Snapchat, they have likely added you back
If their profile shows your friendship status, they added you back
If you no longer see Add Friend next to their name, they added you back