The easiest way is to drag something sharp across the top, perhaps some
car keys, and watch what happens. If the top surface flakes off easily
and seems to want to peel up, it's a CD recordable. If you'd like to be
able to use the disc afterward, there are some non-destructive ways too.
In some cases it's easy to tell, e.g. the color is slightly off or there are
two different shades of silver. The written areas on a CD-R look slightly
different from unwritten areas. A silver CD-R that has been written to
capacity is nearly indistinguishable from a pressed disc though, and some
pressed discs have distinctly visible regions.
You can get a definitive answer with CD-R Media Code Identifier (6-2-9).
Put the disc into a CD recorder and query it. Pressed discs will say
"no information". Some CD recorders might have trouble finding the ATIP
after the disc has been closed, so do some tests with known discs before
jumping to any conclusions.