how to estimate shared_buffers...

On a running production machine, we have 900M configured on a 16G-memory Linux host. The db size for all dbs combined is about 50G. There are many transactions going on all the times (deletes, inserts, updates). We do not have a testing environment that has the same setup and the same amount of workload. I want to evaluate on the production host if this 900M is enough. If not, we still have room to go up a little bit to speed up all Postgres activities. I don't know enough about the SA side. I just would imagine, if something like "top" command or other tools can measure how much total memory Postgres is actually using (against the configured 900M shared buffers), and if Postgres is using almost 900M all the time, I would take this as an indication that the shared_buffers can go up for another 100M...

What is the best way to tell how much memory Postgres (all Postgres related things) is actually using?

Thanks
Jessica