As a kid I was taught to put a bit of spittle on the end of my finger to put on the end of the valve stem to check for leaks. Works every time, no need to position the wheel in any specific orientation.Park the car so the valve is in the top position pointing down. Remove valve cap and submerge the valve in a full glass of water you lift up to it.
A product called SLIME is made for tubes to stop leaks. Bicycle riders use it, search online for more info.
I would not recommend this stuff. It just makes a mess and makes it impossible to patch the tube if you want to at a later date. And NEVER use it in a modern tire. It plugs up the valve stem sensor and your dash indicates a flat tire all the time (if so equipped).