By the magisterial teaching of the Church.
By what means do Protestant determine what truth is that it posits outside the actual words of Scripture. And do not be mislead, despite all the claims of sola scriptura Protestantism does not operate according to it. Rather, it posits specific theological claims, Protestant opinions, that it insists that must be believed and places these opinions on the same plane as Scripture. If it truly worked according to sola scriptura it could not arbitrate between Catholic interpretations of Scripture and those of its own. Sola scriptura does not allow one opinion concerning the interpretation of Scripture to be raised above that of another.
How do they know if they’re being led by the Holy Spirit?
See the Council of Jerusalem in Acts 15. How does a Protestant know that he is being lead rather than a Catholic?
Form a committee and vote on it?
Again, see Acts 15.
“By the magisterial teaching of the Church”
If teaching of the church violates scripture I doubt the Holy Spirit is involved. Acts 15 again was not new but the Holy spirit pointing to scripture in Genesis.
John 17:17 Sanctify them in the truth; your word is truth.
Then they do the Berean thing and Search the Scripture to see if the teaching in in accordance to it. Just as the Bereans did when PAUL, of all people, came preaching to them.
See the Council of Jerusalem in Acts 15. How does a Protestant know that he is being lead rather than a Catholic?
Because it lines up with Scripture. The Council at Jerusalem did not admonish the Gentile believers to do something that was not already established in Scripture, ie, Acts 15:27-29 We have therefore sent Judas and Silas, who themselves will tell you the same things by word of mouth. For it has seemed good to the Holy Spirit and to us to lay on you no greater burden than these requirements: that you abstain from what has been sacrificed to idols, and from blood, and from what has been strangled, and from sexual immorality. If you keep yourselves from these, you will do well. Farewell.”
Nothing there that was not already established clearly by God.
Please prove that Protestants have theological claims that "must be believed" that are not found in scripture.
The problem is the magisterium has only given infallible teachings 5 or 6 times ... So what Roman Catholics hear and are taught are the fallible teaching of men and subject to revision and change
That is simply another strawman (and see 12 above), as due to the erroneous RC premise that ecclesiastical authority presumes and requires ensured veracity, RC's charge that lacking this, under SS no magisterial office can settle disputes, despite Westminster affirming that office to do so, while if it does then they claim that is placing opinions on the same plane as Scripture.
However, that is simply not the case, as one can require obedience without presuming ensured veracity, as did the OT magisterium, dissent from which was a capital crime. Likewise civil authorities have power to physically bind and loose. As do parents in a family. But none of which can claim that all their judgments are equal with the wholly inspired word of God, and SS statements cannot, but which Rome effectively claims.
The veracity of the OT magisterium was based upon the premise of consistency with the written Law, and likewise under SS the veracity their claims must rest upon the weight of Scriptural substantiation in word and in power, not the novel and unScriptural premise of ensured perpetual magisterial infallibility as per Rome (and basically in primary cults).
But RCs argue that an infallible magisterium is essential for determination and assurance of Truth (including writings and men being of God) and to fulfill promises of Divine presence and preservation.
And that being the historical instruments and stewards of Divine revelation (oral and written) means that such is that assuredly infallible magisterium. Thus any who knowingly dissent from the latter must be in rebellion to God.
Yet which is simply unScriptural and untenable in the light of it.