The filament temperature T is a key parameter in hotwire-assisted chemical vapor deposition (HWCVD). Three common methods for the in-situ determination of T are based on the measurement of electrical resistance, electrical power, or intensity of thermal radiation at one or more wavelengths lambda. This work discusses the errors due to assumptions in these methods, primarily when an assumed resistivity rho(T) or spectral emittance epsilon(s)(lambda,T) does not match the sample. Further, a method is introduced to find the temperature of a filament behind a viewport with unknown transmittance, and without the need to have references for rho( T) or epsilon(s)(lambda,T). This method combines multiple thermal radiation spectra at varied radiating power and assumes that epsilon(s)(lambda,T) is independent of T within the resulting variation in T. The combined optical-electrical method is within 30 K in agreement with pyrometry around 2000 K for the real-life filament, and within 20 K of the true T when applied to simulated data of a W filament for which T is known.