you can try writing as a username something related to your friend's account.
For example, let's say you want to check if firstname.lastname@example.org
has you in his list.
Then write as a status: "Hey email@example.com, what's up?"
and sign in several times. Or try to write something that will catch his/her eye.
Try this several times for a couple of days. If your contact doesn't start a conversation with you, he/she probably doesn't have you in his list anymore or doesn't sign in anymore or is plain ignoring you.
Though I have a question, do you think someone who deletes you would necessarly block you? Because I'm suspecting that someone in my list has deleted me (he used to talk to me almost everyday) and now I see him online but he doesn't start a conversation with me... I don't want to get his attention with the trick mentionned above but I just need personal opinions on that