Sample wells are designed in such a way that allows you
to take a look and possibly a sample of your waste
stream. They are typically set up right after your grease,
grit or lint trap.  The man hole is typically smaller than the
lids you have for cleaning (pumping) your  trap. The cover
for your sample well, if you have one, is normally within a
few feet of your trap.

Inspectors look at your sample well  and you should too.  
This is a window to your grease trap soul!  If you see
grease caked to
the walls of yous sample well or you see particulate
floating in your waste stream you probably need to pump
out more frequently.  
Remember, it is better to figure this out on your own
rather than being told by an inspector to do it.  It keeps
them happy and that
should make your life easier as well. (no ticket means no
ticket, no court ,no fine etc)
sample well engineering schematic
sample well