Absolute proof may be too high a bar. But you can gather persuasive evidence with word order, verb form, and semantics. Let's start with identifying those subjects that are noun phrases of clauses.
First determine whether you're dealing with an interrogatory. If so, transform the sentence to the declarative form:
Are you the one? -> You are the one.
Determine whether the verb is in the imperative mood. If so, the subject is you, the audience being commanded or exhorted, and you're done. You'll have to be familiar with relevant English idioms:
Let me help you.
For declarative sentences, determine the verb type.
- A copulative verb links or describes, and the noun linked to or described is the subject. That noun will not be governed by a construct taking an object, and it will either be the only noun or the closest noun preceding the verb.
The land beckoning John was a world unto itself.
As the object of the participle, John, isn't a candidate; land precedes the verb was, so it's linked to world and is the subject.
If all you've got is the one noun, its position won't matter. In
The land was beautiful
or the poetic
Beautiful was the land!
land is what's described and it's the subject in either case.
- An intransitive verb declares a state, and the subject is the noun in that state. Same positional rules as 1.
John lies in his bed.
- A transitive verb carries an action, and in the active voice, the subject performs the action.
John hit the wall with a hammer.
- A transitive verb in the passive voice will have a subject on which the action is performed.
The wall was hit with a hammer by John.
I haven't described how to determine the direction of the action, but you do that by word order and eliminating objects as candidates.
There are no doubt exceptions that don't conform to these steps.
And note that English is highly recursive, so this is only a start. Non-clause verb forms like infinitives and participles may have subjects, and both these verb forms and whole clauses may act as subjects of the clauses in which they're contained. But it is a start.