In that they say they are, and they believe in and follow Christ Jesus. Whether they truly are, we're unable to judge. It's kind of like Republicans: some truly are, and some are actually RINOs; but, if there's a big R after their title and they're registered in the party, they are Republican.
In my experience and observations, there's a more reliable way to guesstimate:
Their fruits.
Those with a genuine heart after God seem to persistently tend to grow toward
LOVING GOD WHOLLY;
THEIR NEIGHBORS AND OTHERS AS THEMSELVES
AND TO DO UNTO OTHERS AS THEY'D WISH DONE UNTO THEMSELVES.
Which, now that I think of it, seems to be a very rare phenomena the older and larger and more fossilized INSTITUTIONAL expressions of Christianity are.