Spam Assasin only checks the mail as it is being delivered into the POP account on the server, the forwarder happens first.
If it's on the same server then the email will be checked as it is delivered into both Inboxes if the forwarder is external then it only checks locally. All forwarded mails just get sent.
If you are forwarding to an AOL account and you forward lots of spam emails this could get you blacklisted for spamming as the message appears to come from your server even though it originated elsewhere. The forwarded email will have headers identifying your server as the sender.