A method of measuring the viscosity of a resin in a bottom-up additive manufacturing apparatus, includes the steps of: (a) providing an additive manufacturing apparatus including a build platform and a light transmissive window, said build platform and said window defining a build region there between, with said window carrying a resin; (b) advancing said build platform and said window towards one another until said build platform contacts said resin; (c) detecting the force exerted on said build platform by said resin; and (d) generating in a processor a viscosity measure of said resin from said detected force.