All you need are some incense sticks (DIY Leak Detector); holding two or three together creates more smoke for easier detection.  Powerful leaks (greater energy loss) will cause the smoke to dissipate and the tips of the sticks to glow.  Slower leaks (lesser energy loss) will cause the smoke to trail away or move towards the leak.  Another little trick is to use a flashlight to detect gaps around the edges of a closed exterior door.  While shining the light around the doors edge have another person check for light penetration on the other side.  If any light is visible you’ll want to seal these gaps to prevent further air leakage/energy loss.