[header] How to diagnose hookworms in dogs [title] Be on the lookout for diarrhea or vomiting. [step] A hookworm bites into the bowel wall, which can cause pain and stomach cramps. The dog is also likely to have diarrhea that is tinged with blood. 
[substeps] Also, if there is bleeding into the bowel, by the time the blood have passed along the gut it gives the feces a dark color. Some dogs may also vomit from hookworms, and this could contain blood as well.