1) When you try logging into your AdSense account, it shows a message that your account deactivated... 2) You get a mail from AdSense saying the same... 3) Blank spaces appear in place of your ads... You will know it's banned if 1 or more of above points holds good
You know you've got banned from an email from adsense. The subject is "Your account has been suspended". When you try to login to Adsense CP, you'll get an error that you are not approved yet.