I generally ask who and what inorder to identify subject and object of a sentence. For example,
e.g. Jacob told the truth
Who told the truth. It is Jacob.
What did Jacob tell. It is truth.
But look at the foll sentence :
smoking appeared to be the cause of the blaze
I cannot ask the qeustion who ... in this case.
So I ask
Smoking appeared to be the cause of what ? (blaze)
what appeared to be the cause of blaze ? (smoking)
How to identify subject and object in such cases ?
(A person) smoking was the cause of the blaze.
In this case a person or persons is the understood subject because animals and things do not smoke.