How would someone know that?
The same way you would know whether or not what you hear fom anyone (unfortunately even priests and Catholic instructors of many kinds) is rooted in Catholic Doctrine, Dogma and Tradition, fact check. Read and study Chuch teachings and catechisms and the saints. I sponsored someone in RCIA once and remember the sister teaching the class said Catholics no longer need to read or study the catechism. That was bothersome.
Traditionally Catholics rely on the current Magisterium to guide us in authentic interpretation of Scripture and Tradition.
It is true we do rely on the current Magisterrium to guide us, but we do not want to leave behind what has been taught to us from the time of the apostles and it is our responsibility to study the faith ourselves and even our Catholic saints of the past.
Many Protestants emphasize Tradition, but obviously reject the Magisterium as authentic interpreter of it. Unfortunately this leads them into contradictory interpretations of Tradition. Protestants don’t submit books for Imprimatur, as Fulton Sheen did, nor do they maintain a relationship between their media organization and the local Ordinary (like EWTN).
Again this is also true. Protestants have a different way of looking at things but that would be a different topic since we are discussing Catholics.
I have not read Dr. Marshall’s book and you are right he does not have an Imprimatur but he does have a forward from a bishop in very good standing with the Church who also has an excellent book out that speaks similarly of the crisis happening in the Chuch today.
Christus Vincit.
An Imprimatur in a book only means that according to the person examining the book it can be printed. It is not an infallible statement, it is not coming from the Pope or all the bishops. It is usually from a Censor Librorum (cleric) that the bishop trusts to examine the book, so many times it isn’t from the bishop specifically.
As far as vidoes and podcasts and blogs, they are constantly changing and growing in number so it would be very difficult for the Church to give any kind of approval or Imprimatur for what is happening on the internet. We tend to focus here a lot on Dr. Marshall but there are a lot of Catholic youtubes and blogs out there.
Speaking of EWTN, (as per a post in this thread) some see EWTN as an enemy of the Chuch. I do not, I like EWTN very, very much but Mother Angelica herself spoke out frequently of what she saw was a crisis in the Church.
I guess bottom line is pray and ask God to lead you and make sure that what you hear and understand is Catholic truth.
Many times it is not what we hear that upsets us but the messenger or how the messenger presents it. My suggestion is if you don’t like the way one person discusses the crisis in the Church try listening to another person.
If we didn’t think there was a crisis or problems in the Church we would be denying what Christ said when He said, if the world hates me they will hate you.