the best way to now that is from their writings
I am sure you believe this, but we have far more substantiation in their teachings. We have liturgy, prayers, and the testimony of those who were taught by them. So we are not limited to what a certain Apostle wrote (most of them did not write anything that remains).
We have followed the Apostolic command:
2 Thessalonians 2:15
15 So then, brethren, stand firm and hold to the traditions which you were taught by us, either by word of mouth or by letter.
This gives us an advantage because we have more testimony that those who are separated from Sacred Tradition.
Then by extrapolation (all things being equal, with no speration) all things could be ruled by scripture, that one could just as easily go and teach all nations what is written.
The fact that there is no contradiction does not equate to the Sacred Tradition having no value.
And yet it is interesting that some new heresies are declared just as after some “current…application” by the magisterium
Perhaps you can give an example? I think history will testify that the declarations are in response to heresies. The first Magesterial council in 50 AD was in response to the Judiazers. The early Church also responded to a variety of heresies, gnosticism, arianism, etc. The proclamations are to protect the faithful from falling into error, because if they do, they will pass through the gates of hell.