I have reason to believe a certain cafeteria I frequent puts laxatives
in their food. Does a test for laxatives exist and if so, what is the
simplest way to text for the presence of laxatives in a sample of food
(such as enchiladas, pasta, corn)? I have experience performing
chemical tests in a scientific laboratory.

"Laxatives" is a very broad term. If you wanted to test for a certain
specific chemical compound, this might be do-able. But even a
sophisticated lab needs to know what substance is being sought in a
sample. There are hundreds, maybe thousands of substances that can be
used as laxatives.

I can't think why a food establishment could want to put laxatives in
the food. Not only would this open the owners up to lawsuits, but it
might make the customers ill and unable to return to buy more food.

To answer the second comment: because its a college cafeteria in the,
per capita, fattest county (East Baton Rouge Parish, Parish is
Louisianan for County) in the (by far) fattest state. Its not
neccessairly an issue that keeps me up at night, as I've heard rumors
ranging from "they spray butter on the salad greens to prevent
anorexia" to the laxatives suggestion. But I'd like to perform the
test, if its simple.

The fact of the matter is, the majority of students I've interviewed
say they have problems with loose stool 2 out of 5 times they eat at
the cafeteria.

To be more specific, what is the test for fiber content? It would be
easy to mix benefiber, a tasteless fiber that can be mixed with
anything, into pasta flower, cookie dough, or soup.

Also, are there any other cheap, tasteless laxative compounds commonly used?

Have you thought this through? If a school were found to be
adulterating food, those responsible would almost certainly face jail
time. That's a mighty big risk to take for no apparent reason. What
possible motive could there be for dosing students with laxatives?
It's not as if the school will profit from an increase in bowel
activity. If anything, the additional toilet-flushing will probably
cost the school, since there will be greater use of water.

Loose stools in a person after eating in a public place like a
cafeteria is probably due to bacterial contamination, not laxatives.
Unsanitary food handling, and workers not washing their hands can
spread fecal and skin flora onto food. Do the food handers always wear
gloves, and do they discard the gloves after handling non-food items,
ike emptying the trash? Cutting boards, counters and dishes may not be
getting properly sanitized either.

  Besides, to add enough laxative to affect people, and not be
detected would be very expensive!

If it really is such a general problem, complain to the person in charge.
If that doesn't work, talk to the public health authority that
inspects the cafeteria.