The part is laid down, web horizontal to the floor and either a string or I guess a laser could be shot across from one end to the other. Measure the distance the beam is cambered. Once the beam is stood up, the weight of itself may make the camber less, thats why it HAS to be checked in the unloaded position. It can not be correctly checked if its installed. (we've had third party inspectors try that with a cement floor poured onver cmabered beams and they said we never cambered the piece.)