Simple procedure for showing presence of milk proteins in clarified butter (ghee)
A classic test for protein is Biuret solution, using readily available reagents:  sodium hydroxide, copper sulfate and potassium sodium tartrate (Rochelle salt). The test is not specific for milk protein, but what other protein would be in pure butter?

Use caution with the chemicals -- the first listed is quite caustic, and the second is poisonous. BTW, save some leftover Rochelle salt if you'd like to make a piezoelectric crystal.