The SY has green bits of camouflage which partly cover the deck, and which you can see on the sides of the superstructure even if the angle is head on. Also, if he's running in a straight line he's a noob anyway, you should watch him turn because he should be zig-zagging.
But yes the SY should basically have a green deck front and back, the Yammy is silver.
I never had a problem myself, but if you REALLY can't tell I recommend getting some modded sprites. I used this site for some just to make other ships look cooler:
in the IJN section under BB there are some Darker Yammy and SY sprites that should solve your identification problem. Pick one you like that's different enough and just change it.