How can I find the perfect handlebar height?
Raising handlebar height is very easy to do, but you may need parts. 

There are a number of ways to raise a handle bar. You can also move the seat forward, use a shorter stem or narrower bars to get a similar effect.  Note however than any change can have an effect on handling of the bike.  

No parts ways: 
Spacers: If there are spacers above the stem, put these below the stem. 

Seat: Moving the seat forward will have raise your torso. 
With riser bars you may be able to rotate them, depending of sweep. 

Narrower bars - you can trial narrower bars just by moving the grips and shiftier/brake levers inboard. If you like it, you can cut the bars down. The current trend in MTB's is wide bars and short stems, so its almost certainly possible to go narrower. 

Parts way: 

Stem - stems are very easy and quick to swap. When buying a bike, the shop should be prepared to swap stems around and let you try til you find one you like. To get yourself sitting higher, a steeper angled and/or a shorter should be tried.

Bars - raiser bars are a good way to go, they come in various heights. 

Stem riser - these bolt onto the steerer tube and effectively increase the length