How can I tell if a message was sent to just me or to a mailing list?

Gmail displays arrows, or 'personal level indicators,' next to messages in your inbox so you can tell if a message was addressed to you, a group, or a mailing list. A single arrow (>) appears next to a message when it is sent to you, and a group of others. Double arrows (>>) indicate that a message is addressed only to you. Arrows won't appear next to messages sent to a mailing list.

Here's how to enable personal level indicators:

1. Log in to your Gmail account.
2. Click 'Settings' at the top of any Gmail page.
3. Select 'Show indicators' in the 'Personal level indicators:' section, and click 'Save Changes.' (If you'd like to disable personal level indicators, just select 'No indicators' and click 'Save Changes.')

https://mail.google.com/support/bin/answer.py?ctx=%67mail&hl=en&answer=8156