Are you trying to measure trace residue of FBS, or just the normal amount that is used in cell culture media (typically 5-20%)? If it is the former case, there are commercial kits for this purpose. Here is one example:

https://www.virusys.com/bovine-serum-albumin-bsa-antigen-capture-elisa

If it is the latter case, then you can use a standard protein assay such as the Bradford or BCA assay, since there is a huge amount of protein in FBS. You can use full-strength FBS as the standard.

thanks

Adam most likely wanted to suggest a BCA assay.  However, be aware that some components of your sample might interfere with the protein determination by Bradford or BCA.

https://en.wikipedia.org/wiki/Bicinchoninic_acid_assay

Thanks, Michael. Correction made.