The application of various functional protein additives in the meat industry is still increasing. Soy, milk and egg white proteins are applied to various types of meat product to improve their structural properties or as good substitutes for meat and fat components. It is common practice to add these preparations to reduce production costs. New methods are needed for reliable detection of small amounts of these additives in complex and processed food matrices regarding the potential for food adulteration and allergen detection. In this work, a mass spectrometry method was implemented for the detection of soy (Glycine max), milk (Bos taurus) and egg white (Gallus gallus) in smoked and cooked pork sausages. Specific peptide markers for glycinin and β-conglycinin, α-S1-casein, β-lactoglobulin, ovotransferrin and lysozyme C were monitored in the sausage samples. The proteins were detectable at the low concentration of 0.53% (w/w) of a given additive, which corresponds to a limit of detection of 0.14% for milk proteins and 0.45% for soy and egg white proteins in the final meat product. The results were validated by the use of synthetic peptides labelled with stable isotopes. Considering the universal nature of the presented peptide markers, the method has the potential to be implemented for other complex and processed foodstuffs for authenticity purposes.