Kosher in a place catering to a yeshiva crowd means mutter to the standard generally accepted among that crowd.
I'm not sure I agree with this. Perhaps.
But because there are so many people who don't know enough about kashrus making that claim based on their ignorance of those standards, it may very well be required to ensure the guy is familiar with what is neccesary to meet those standards. Either by asking him some basic qs, or asking someone who you know is familiar with kashrus who knows him, if he knows whats neccesary.