We report on fundamental mode content measurements performed on single-frequency large mode area fiber amplifiers based on Er:Yb-codoped multifilament-core (MFC) and step-index pedestal-design fibers. The mode content of the amplifier output was analyzed with respect to free-space TEM modes using a nonconfocal scanning ring cavity. At power levels of several watts, the commercial step-index pedestal-design fibers with respective core diameters of 23 and 25 m showed a mode content exceeding 80\%. However, a mode content of up to 95\% could be obtained with an MFC fiber also with larger core diameter of 31/28 m in hexagonal shape. To the best of our knowledge, we utilized the nonconfocal scanning ring cavity technique for the first time in the wavelength region of 1.5 m and additionally for the first time on fibers with pedestal as well as MFC design.