Do you use a commercial test kit? Then the positive control should be included. For a home-grown test, you need a serum sample that you know is positive, that is, a sample from a diseased animal. Usually, they are around in some freezer, ask your colleagues.

Use either an endogenous soluble sample known to contain the protein you are detecting or a purified protein or peptide known to contain the immunogen sequence for the antibody you are using. A positive result from the positive control, even if the samples are negative, will indicate the procedure is optimized and working. It will verify that any negative results are valid.

Alternatively, check the antibody datasheet which will provide a suggested positive control.

You can as well check the customer review for antibody.