How can i make sure a water tight cap is in place
step 1: Rewrite the problem as the solution
==> fill the system with water and if it leaks it is not in place.
Step 2: make it realistic
==> fill it with air and if the pressure goes up the cap is in place, if not its not.
Check - www.triz40.com - for pattern based problem solving (might be not the best site to start but OK)