We present a method to characterize and optimize a tapered-amplifier laser system (TALS) by its spectral quality through a long multi-pass rubidium absorption cell. A thermal vapor cell is used to measure the non-resonant spectrum of TALS, including the broadband amplified spontaneous emission (ASE), which is its main spectral noise. This method gives a simple quantified measurement to optimize various working parameters of a TA, including current and temperature online. It can as well be used to compare various TA chips based on their usage time during our precision measurement experiments. The results of this method are compared and found in sync with traditional methods of Fabry-Perot cavity and beat measurements. Such characterization and optimization are important for noise control in atom interferometers, atomic clocks, and other atomic manipulations. It can very well be used for investigating nonlinearity and ASE inside amplifying chips and can be utilized in other applications of ASE using bioimaging.