How do I know if a buddy is blocking me?

Greetings!
Finding out if someone is blocking you is fairly easy.  The first step
is to sign up for a second instant message account which no one else
knows about.  This can be done here:

https://my.screenname.aol.com/_cqr/registration/initRegistration.psp

With this new name log on and create a new buddy category, such as "Am
I blocked".  Add the friend in question to that category.  Once you've
done this you need to link the two screen names.  Instructions on how
to do this can be found here:

http://www.aim.com/help_faq/using/win/acct_linking.adp?aolp=

Once the names are linked watch that Am I blocked category.  If your
friend signs on and their name appears there but not in the regular
place it means that they have you blocked.

If you have any question or problems please request a clarification
before rating my answer and I will be happy to help.

Best,
djbaker-ga

Or, you can use your own 'gut' instinct. If you say hello to your
buddy (say, on Yahoo) and they never reply, chances are that either
they have blocked you or they don't want to chat with you (temporarily
or otherwise). Either way, the end result is the same.

On AOL, if your buddy never seems to show up online anymore, then the
same basic idea applies- unless the person really isn't around (maybe
busy with work, on a trip, etc).  But why would you want to go
snooping around to see if your friend has blocked you. If they are
simply gone for a while and then return later on, I'm sure you will
see them on and you can continue to chat. If they're never around
anymore, then there must be a reason for that and you shouldn't snoop
around to find out why. Maybe your friend simply wants to take a
break. Maybe they don't feel like chatting for the moment. And, maybe
they don't want to chat with you anymore.  But that's just a fact of
life.  If they have blocked you, maybe they weren't worth chatting
with in the first place.

Both of these answeres are true, but also lets say this is the situation:

for AIM: 
There is a option where only the people you have on your list can see
if you are online or not.

Lets say we have Person A and Person B... 
these both people added each other, but A was mad at B, so 'A' changed
the settings to where only people on his/her list can send messages to
him/her.
they can do that by going to the AIM's Preferences, Privacy---

they can pick "who can contact" them and have a option of three things:
-Allow all users to contact them
-Allow only users on their buddy list
-Allow only the users below...


but it depends on how much they know about AIM, its different for each
Messenger Program...

you can always ask a friend to add X person to their list.... and see
if they get online or not.

The easiest way is to create an inconspictous second account with a
username that has no relation to you whatsoever.  Use this account to
try and contact the buddy in question.  If successful, they have
blocked your other account.

Another sign is if their username has a comment which seems outdated. 
This is a usual sign that you have been blocked.

I hope that I answered your Question.