The targeted shaping of femtosecond pulses in 4f pulse shapers is complicated by, among other factors, the crosstalk between adjacent pixels of a spatial light modulator (SLM). Current methods for the crosstalk evaluation require setting up a different experiment, which is highly inconvenient. Here, we propose a simple procedure to extract the pixel crosstalk within the standard SLM calibration used in pulse shaping. The calibration is based on an analysis of the contrast of a periodic modulation in the spectra induced via SLM. We demonstrate the calibration procedure on a liquid-crystal-based SLM and show that we attain a constant crosstalk effect represented by a Gaussian function with σ=1.0 pix over a broad operational range of the SLM.