You'll know a post is from a moderator when that user has a diamond (♦) next their name: for example, me right now.
But in general, the idea is that, by knowing how active a user is on the main site, you should be able to determine how well the user knows the main site when answering questions about it. It's not a 1:1 match, but it works for most meta purposes.
This was also discussed on Meta Stack Overflow, where Robert Cartaino said:
It seems like the more participation you have on the PARENT site, the more abilities and bigger the role you should have in the discussions about that site. Creating a separate reputation score for META activity means that only frequent META USERS would have the leading roles... not necessarily the same people who actually use the site (i.e. the stakeholders). Having one meta score (for using the ACTUAL SITE) seems like a better system.
One other thing to consider is that it frees up people to participate without affecting a reputation total: people can down-vote bad ideas and participate more without worrying about their reputation being affected. On child metas, where there are already low activity levels, killing as many barriers to entry is important.
An answer by Rebecca Chernoff and a blog post by Jeff Atwood touches upon this aspect and others.
But there's some confusion because Meta Stack Overflow, ostensibly the meta for Stack Overflow, does have its own reputation system, and the suggestion from the other angle—eliminating rep there and pegging it to Stack Overflow—comes up often, too.
A detailed answer on why there is this discrepency can be found on the question Should reputation from SO carry over to Meta?:
Each site's meta (other than this one) is designed for site-specific discussions about things like:**
- Defining a site's culture
- Determining each site's tolerance for certain types of questions
- Discussing tagging practices, moderators, etc.
And on those sites, it makes more sense for the meta site rep to mirror the parent site rep, since **the most "trust" to define the site's culture should be assigned to those that have earned the most trust on the site itself; they are its culture.
This site, meta.SO, continues to serve that role of discussing the culture and goals of StackOverflow.com (and that of Area 51,) but it also serves an important larger need:
It is also the meta site for discussion of the unified engine that will drive both the trilogy sites and the SE 2.0 sites, addressing things like:
- Feature Requests
- Questions about the system as a whole (ads, rep-gaming gaps, re-posting policy, etc.)
On a site about the engine, it does not make as much sense to make the reputation mirror one or some of the other sites.