But who and what measure could be used to determine for one to make such a strong accusation
Obviously at some point there has to be some trust. For someone born into the faith, there is assumption that is their belief system. Now if people know they very obviously do unsikh like things openly or openly declare they are no longer following or serious about it makes it easier. But outside of that there is assumption that unless otherwise indicated, those born into a faith are followers of that faith.
For converts it's a bit more complicated. Often people experiment with different religions until they find what suits them and / or it's just a temporary fad. If it's not something your family practice and you weren't born into and raised in that faith then before undergoing a serious commitment I can understand why they require at least some show on part of the convert that they are serious... hence the name change. It simply shows commitment. For those born into it, family is the support system.
As for those who drink alcohol after their weddings, it's between Waheguru and them. What more can we do? What we can do is make some checks and balances to ensure it's not a mockery but some things are out of our hands. At least those who drink alcohol usually they are still following Sikhi and back at the gurdwara the next week again so it's not that they are not following Sikhi they always have chance to improve.
But for a convert it's difficult because so many decide to follow a faith and then later decide it's not for them. So some show of sincerity is not a bad thing.
But for those who OPENLY declare a religion that is not Sikhi, how can they undergo a religious ceremony which they do not believe in, and make commitments in front of our Guru which they never intend to keep? This is why outright interfaith is frowned upon. It's a disrespect to SGGSJ to have someone make commitment to follow Sikhi path and make SGGSJ centre of married life and then blatantly toss that out the window on the way to the reception.
But notice I am NOT against interfaith marriage. Just interfaith anand karaj.