The company I work for manufactures Clear Blue -they test for Beta HCG. Human chorionic gonadotropin . Old days they would inject urine into a frog or rabbit and then dissect the animal to evaluate its reproductive tract hence the old saying the rabbit died. Without a long drawn out answer the test looks for human hormones. Cows do not secrete that BETA HCG but do secret another hormone.
There is a company that makes a ruminant pregnancy test. BioPRYN evaluates the blood of ruminants for a protein called Pregnancy Specific Protein B (PSPB). PSPB is produced by the placenta, and therefore pregnant animals will have the protein in their blood. This test is specific for all kinds of ruminents- Sheep, Bison etc.