You should be able to place your pump with the inlet facing up. If you wouldnt mind post what type of pump you have please.
For leak testing your best bet is to assemble, fill-n-bleed, and run your watercooling setup externally for 24-48 hours. I would also consider placing all the components in as close to the relative final internal positions as they will be. In short simulate the watercooling as it will look inside the system while it is still outside the system. Place all components on paper towers or something like paper towel since it shows when is wet fairly easily.
Once you are done with that you will need to probably partially disassemble your system to put it into place. Once everything is reconnected run it again, this time for around 12 hours without turning on your computer. A relay will probably need to be used if you are using a pump which runs off the power supply.
By the time you have done these two steps you should be fairly confident about leaks.
Keep a couple things in mind.
1. Every time you move your system around it will put different stresses on the connections. Each time you do this make sure to check each and every connection visually and by touch.
2. Not all leaks are of the fire hydrant variety. Make sure to do your best to scan and feel for slow leaks, which while less dangerous to your components, can over time become just as much of a problem or develop into larger leaks.