I suppose my point is we have no conclusive idea who is a valid priest anymore.
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This is not true. If a priest is descended from an unbroken line of bishops ordained and consecrated in the true rite, who had the knowledge to perform the ceremony correctly, then there is conclusive proof that he is a valid priest.
I've even seen arguments that Archbishop Lefebrve's ordination may be in question due to it being performed by an alleged freemason. Doesn't the fact that Fellay has accepted most of Vatican II place them in the same boat as the FSSP?
The argument against Abp. Lefebvre has been debunked numerous times, but without getting into the details, I think you need to make more distinctions between pre-Vatican 2 and post-Vatican 2 Catholicism. So, Abp. Lefebvre was ordained and consecrated during the normal times of the Church, and no one raised any doubt about his holy orders. Therefore, it is implausible for anyone to do so today, apart from the other reasons their arguments are absurd.
On the other hand, before Vatican 2 Pope Pius XII definitively settled the question of the matter and form of holy orders. He said what words needed to be said for a bishop to be consecrated validly, and what it had to contain. He used his supreme authority as pope to bind everyone to this. Then, not even 30 years later Paul VI comes along and throws that completely in the garbage, and rewrites the form entirely, in a formula that doesn't even contain the elements Pope Pius XII said it needed to have for validity.
Obviously such a travesty can have no presumption of validity. Besides, if you don't trust Paul VI for your Mass or other sacraments, why would you trust him for holy orders?
As far as the fruits of holiness, that is not evidence of the validity of a sacrament. The Church teaches what is necessary for the validity of a sacrament, and they are matter, form, intention, and (for most of them) a priest or bishop as the minister. If a sacrament doesn't meet those criteria correctly, as laid down by the Church, then it is doubtful or invalid. You can't argue that a sacrament lacking the proper matter, form, intention or minister must be valid because it appears to make people holy. Besides, you are not God and cannot read those people's hearts, so you really are not in a position to say how holy they are. If people there truly have have piety, it must be due to some other cause, such as praying the rosary, devotion to Our Lady, mortification, or other things.