Your own maxim
Test a rule from your everyday life with Kant's questions — including the formula of humanity.
The absolute worth of the person: what has a price can be replaced — what has dignity admits of no equivalent.
In the “Groundwork” Kant distinguishes dignity from price: things have a market or fancy price and can be exchanged; persons, by contrast, have dignity — an inner, unconditional worth. The ground of this dignity is autonomy: the capacity to give the moral law to oneself.
From this follows the prohibition of instrumentalisation: a human being may never be used merely as a means. The idea remains influential today — for instance in Article 1 of the German Basic Law and in the “object formula” of constitutional doctrine, which forbids reducing people to mere objects of state action.
Source: Wikipedia: Dignity