The earliest citation of “to suss” as meaning “to figure out” is commonly given as Queen magazine 1966, as here “Suss out”
Well I have sourced that exact issue and here link the cover and the page it appears on, under “Pop Scene”. It will be seen that it was in fact Ronnie Lane of the Small Faces who is quoted as using it.
edit: Well the imgur link doesn’t seem to be working, no idea how to fix it.