Check to see if you have enough by taking your largest knife, (12") and
stand it perpendicular to the wall so the edge of knife is where the
mud has been placed. With adequate light, a quick glance from the
shadow side, you'll be able to see very quickly where the high/low
spots are.

One tip I picked up is to work with thinned mud. Many times it harder
to control on the hawk and knife edge, but it flows off of the knife
easier than straight out of the bucket. You'll have to experiment as
to what works best for you.

I don't subscribe to the "NO-SANDING" thought rather, sand just enough
to remove the obvious bumps and ridges. For what it's worth, if you
plan on leaving the walls smooth, I'd stay clear of the mesh sandpaper,
as it can cause more tiny ridges than the smooth paper will. If you
texture, either will work, the mesh I believe is more aggressive. A
poll sander works well, save your ROS for wood working. Like another
poster mentioned, feeling with your hands will revel many details,
where you need additional work/mud/sanding.

Here's a site that's has some good information:
http://www.drywallschool.com/protips.htm mostly free. I am not
affiliated with the site, just found it interesting.

Good luck to ya!

Darwin