How do I check to see if an email is in the subscriber list...It's not like getting the full active subscriber list and then finding the account in that is it?
If you're looking to see if a subscriber is on your list, you'll want to go to the Lists & Subscribers tab to see the "Manage Subscribers" page. Next, enter the subscriber's name or email address in the search field above the lists and click Search. The help article here gives more information as well: http://help.campaignmonitor.com/topic.aspx?t=169
If you're searching for a subscriber email within a campaign, you'll want to click on the campaign in the Reports tab, then select "Recipient Activity" from the right-side tool bar and then search for the subscriber's email. You could also go directly to the subscriber's snapshot profile to see if that campaign is listed as being sent to them.
Hope that helps!
If you're talking about the API (you are in the API forum), i would say there's two main methods:
1. When you know which list you're talking about, you can ask to get the subscribers details. If the subscriber isn't in the list then the defined error (in the docs) will be returned instead.
2. If you're not sure which list, but you know the client, you can ask for all the lists to which an email address belongs.
Hope that helps :)