Is there an easy way tell who is a moderator? Is that what the diamond after the screen name means?
Yes, the ♦ next to a user name.
You can also see the full list of moderators by visiting the appropriate page.
Note that each SE site has its own set of moderators, so, for example, while I am a moderator here, I am a normal user like you on other SE sites.
In addition to the moderators you see in that page, there are Community Managers, and they have moderator powers (plus some) on every SE site.