How to check if a list contains members or not before executing an email

In the schedule tab of a smart campaign or in the 1st tile of an emial program it should tell you how many leads the batch campaign is set to affect. Or are you looking for something else?

No, I am looking to only fire off an email if a particular list is empty. So, I am trying to figure out how to check for that in my program steps? Does that make sense?

Yes, that's clear.

Smart list filters and triggers all refer to a lead and its values. In this case I think Marketo would need different types of smart list filters that are not checking for lead properties but are checking for list properties.

Maybe if you can tell us a bit more about the campaign (like why does the list need to be empty and how do leads move in and out of the list) then we might have alternative suggestions.

Just wanted to share that this is also possible via the APIs:
http://developers.marketo.com/documentation/rest/member-of-list/

Thanks for the help. Here is what we are trying to do

Step 1: If the incoming lead's WebApp_BJN_GroupId (this is one of our custom lead fields) exists in a list A - do NOT send the email

Step 2: If it does not, send the email and add them to list A

Bascially, we are trying to lookup a value in a separate list before executing an email. Does this help. @Murtza - how can I we incorporate this API to work for us? Sorry, but I am not a developer and this is probably too complicated for me :)