For example
if you live at home with parents or siblings, when your alone at home, let the bathtub filled with water, as you would take a bath, sit in there until you hear them coming near, then you could get into a position, where you lie stretched with you face in the water.
But my opinion: i would not recommend to fake ctb attempt, just to verify who is a real friend. You will be send to a mental institution and get drugged with stuff which will make everything worse, thats not worth it.
Another option could be to fake your death, by let your parents post things on facebook like you have been died and you could "play hide and seek" with your friends, by not going outside the house and not log in social Media accounts for maybe 1 week. Perhaps you will see who will react in the correct way and who doesnt.
But if you emerge, you could lose some friends, which would be angry about your test.
A test for who is a real friend and who does not has been made by a person who claimed, she had won in the lottery, then she had recieved some comments on Facebook which revealed her fake friends.
Here's the link(in german):