Just to clarify, that if the person who we think has reverted is someone with whom marriage might be considered, then generally the way to see, is to have that person in touch with those who can teach them Islam, and can help them. Then usually, people leave them to settle into the deen, and wait and observe them for a period. Some might wait a year or two, some might wait six months or so, by which time they will know if that person behaves like a Muslim, lives like a Muslim, prays, fasts etc. Only when they are completely satisfied, that that person is serious about Islam, has made all efforts and taken steps to learn, and practices, then is any next step taken.
It is a very serious matter. There are instances of people having said the words of the shahaadah, and the other side thinks, good, they're Muslim, marriage takes place and actually, they only said it to get married, and end up with a "marriage" that is not Islamically valid and is a sin.
There was a comment above about hiding good deeds. I'm not sure this applies when it comes to the fard, such as daily prayers, Ramadan fasts etc. And Allah knows best.