When sending a campaign, you can review your audience once you've uploaded or added your contacts to the campaign.
To review your audience, click on the View Audience button
You will find the View Audience button near the top-right of the screen.
After clicking the View Audience button, you'll see the contacts that will be receiving the campaign message
You'll have the option to exclude the contacts from here as well.
To review your excluded audience, click on the View Excluded Audience button
After clicking the View Excluded Audience button, you'll see the contacts that will not be receiving the campaign message.
Some reasons why you will see excluded contacts:
They have opted out of receiving messages from you
You have blocked them
You manually excluded them
You applied settings in the campaign to exclude certain contacts, such as Exclude Contacts with Open Conversations or Exclude Contacts who recently received another Campaign