We don't expose this information - and I would be a bit concerned about doing so for a couple reasons:
1) I am not convinced that the people @ mentioning would actually check a list (granted we could attempt to do so automatically if we implemented it)
2) We would need to distinguish between following (very likely to read) and tracking (will read) since if a user is following a thing but you really want him to see it, the mention may still be important
3) Depending on the usage - users may prefer that others not know what they are following. I know this is potentially cultural, but where we can lean towards user empowerment, we try to.
A feature we do want to implement is doing a better job of consolidating duplicate notifications (so for example if you mention me and share the same doc with me I get 1 alert that indicates both actions)
Here's the advice I would give for now:
1) if someone wrote the post or has commented, they are "auto-tracking" it, so all replies end up in their communications - no need to mention
2) the mention is a social signal that says "hey, I think you need to see this" - which is great - but people should use it wisely or they will end up like the boy who cried wolf
Hope that helps.