Know if a mail was sent just to you or a group (before opening it)

• Login to your Gmail or Google app account.
• Click the gear icon at the top of any Gmail page.
• Click 'Mail settings'.
• Go to 'general' tab
• Go to personal level indicator section.
• Click 'show indicator', and as the description reads your mail listing will now "Display an arrow ( › ) by messages sent to my address (not a mailing list), and a double arrow ( » ) by messages sent only to me.”
• Click "save changes” button at the bottom.

Now

• If a mail is sent to you only, it will have double arrow ( » ) in the inbox next to this mail
• if it's sent to more than one person (including you) then it will display a single arrow ( › )