A presumptive test will change the color of the stain if semen is present. The problem with this test is other things will also make it change color and should not be considered definitive proof.
A microscopic test is also completed. This test will search for the presence of semen, field by field.
A protein test will also be performed, these three tests together will conclusively tell you whether your sample contains semen or not.
Some of the drug store variety of these types of tests are just a presumptive test. Also many kinds of chemicals that are used can make it impossible to later use as a DNA sample. If you want to test an article yourself at home, please cut out a small corner of the stain, and test it. Please do not dump the whole bottle of chemicals on the article, as it may cause you to never know who the donor was.