Look for any slight stains or discolouration on your ceiling/walls.
Using the grey lead pencil, draw a fine line around the outside of the stain. Tip: If using a ladder, always ensure it is securely placed on an even surface before climbing and never stand on the top rung.
Keep an eye on the stain to see if it grows.
If it does grow, what might have caused it. Did you use the shower, turn on the heater, or was there a storm?
Once you've narrowed down the cause of the leak, call the appropriate tradie to check it out.