Using live traffic is a proven method. I have used a single node sending data and the gateways in my area to compare a unknown antenna (RAK 5,8 dBi if I remember well) with a known one (dipole). Using this method I was able to verify the gain of the RAK antenna within 0,1 dB. I have posted this years ago on this forum.
Its a flash-back in history but this post explains how I verified the antenna then: The BIG and SMALL ANTENNA topic part 2 - #267 by pe1mew