Consider the following sentences that sparked the confusion I now face.
El sol secó la ropa. The sun dried the clothes
La ropa se secó al sol. The clothing dried in the sun
In the second sentence, se is used before the intransitive verb secar.
Researching why, I found the link Reflexive pronouns and intransitive verbs. In which the following explanation was given.
Unlike English, if you say "la ropa secó...", any Spanish speaker would wonder "secó... ¿qué o a quién?" because this verb is not used as intransitive, even if you omit the direct object. That "se" turns the verb into intransitive and solves the problem. English can easily turn transitive verbs into intransitive by just omitting the direct object, but in Spanish this object is assumed to be implicit if not present.
From the above explanation, my understanding is that se indicates that the verb being used is intransitive rather than transitive. The problem however, is that I can think of a few sentences where a verb that can be both transitive and intransitive is used intransitively but se is not used as in the following.
Llama cuando quieras. Call when you want.
Cuando llegué, la tienda ya estaba cerrada. When I arrived, the store was already closed.
Ella practica a menudo. She practices often.
I hope that I am clear in my explanation that I am not asking about the impersonal se or when se is used to show something is done by accident etc... since these may also go before an intransitive verb. Rather I am asking why/how do you know when some intransitive verbs require "se" to differentiate it from its transitive counterpart and when it doesn't.