Hi Smith,
I found this paper for you;
http://iopscience.iop.org/article/10.1143/JJAP.51.04DG14/pdf
for rule of thumb, you can check the light profile on a screen and based on the peaks and zeros, you can identify the mode. then measure the power of this mode. then bend the fiber and repeat the process many times so you can have an estimation on the coupling between the propagating modes.
I hope that helps,
thanks
Mohamed Abouseif