the signs and symptoms vary (some have reported none until very late)
screening examinations for colon (large intestinal) cancer
http://www.gastro.com/html/colonoscopy.shtml
http://www.cancerhelp.org.uk/help/default.asp?page=2817
If you have just the barium enema, you might also want a DRE
J

It is hard to distinguish early colon cancer from much more common
gastrointestinal ailments such as irritable bowel syndrome.   My
mother's colon cancer was discovered as a result of a bowel obstruction,
but when it gets to that point, it is usually pretty advanced, and the
prognosis in not good.   My mother managed to survive it, and two
additional unrelated cancers, and she died at 80 from cardiovascular
disease, but she was just plain lucky.

There is one simple test which catches many early colon cancers: testing
feces for occult blood.   Your doctor will give you some cards on which
you smear some feces and then return them.  A simple chemical test shows
whether there is any trace of blood.   Of course, many other things
except colon cancer can cause such bleeding, but if it is detected, your
doctors will follow up with more extensive tests to find the source.
Everyone past a certain age should do these tests once a year.

A more definitive test is examination of the colon either by Xrays or
through a fiber optic probe, i.e. a colonoscopy.   There is also a
procedure cally a flexible sigmoidoscopy, but it only looks at the last
third of the colon.   It seems now agreed that the colonoscopy is the
preferred test. These tests look for polyps which have a tendency to
become cancerous.  If found, they are removed and examined.   It takes a
long time for a polyp to become cancerous, so you don't need a
colonoscopy very often.  I think ten years is the usual recommendation
unless you have a family history of colon cancer, in which case it
should be done every five years.   A small minority of cancers of the
colon arise from tissue other than polyps and these tests may not find
them.  But nothing is perfect.

A diet high in fiber and low in fat can help you avoid colon cancer.
Taking one baby aspirin a day may also help.

If you're older than 40, get a PSA test.