Accurate measurement of optical gain is essential to screen materials as viable active media for thin-film laser applications. The net modal gain is typically measured using the variable stripe length (VSL) method, which has been extensively studied for the last few decades. In this work, we propose an alternative method, which we name scattered emission profile (SEP) method, to measure the net modal gain. It relies on the collection of amplified spontaneous emission (ASE) scattered from the surface of the film illuminated by a pump stripe. By using an appropriate setup, the new method results in a significantly faster measurement of net modal gain, while simultaneously providing a more accurate gain value. The setup and algorithm to extract the net modal gain are detailed in this paper and are demonstrated on lead halide perovskite films. The influence of the stripe length on the measured gain value is shown. Gain measurements performed over two different perovskite films, fabricated either via spin-coating or thermal evaporation, confirm the broad applicability of the SEP method. Finally, we show a quantitative comparison of the SEP method with VSL measurements and highlight the advantages and shortcomings of each method.