I'm finding this difficult to describe because I don't know the terminology but here goes:
I know the pieces of balsa going fore and aft inside the wing that give the film covering shape are called "ribs" so I am going to call the pieces of balsa going side to side in the wingtips that give the wingtips shape "riblets".

Thus, the balance point is at the first forward "riblet" (You'll need a helper)
This should be the approximate location of the wing spar - most of my planes balance at or just forward of the wing spar.