How to know who were blocked because of communication limit

I don't think there's a way to do that easily but you can run a Smart Campaign to look for leads who were sent a certain amount of emails in a certain amount of time (based on whatever your Communication Limits settings are in Admin.) See below for an example filter. Before the Campaign is sent out, you can view all blocked leads from the Schedule tab (which includes but is not limited to leads blocked due to communication limits).

The other way that you can view is by going to the Results tab of your send, and it will show if the email was sent, or if they weren't sent the email due to the communication limits. 

Dory, There is no Results tab in an email program.