I always judge it like a chip and if the sand is soft and fluffy I take a harder swing at it as oppose to hard and wet sand a softer shot is required.

If it's a green side bunker then use a full flop for fluffy sand which will elevate it onto the green and if it's harder then use less club judging on how hard/soft the sand actually is and how far away the pin is

Honestly, it's a stupid answer, but for me, I find that it's just practice. I started with hitting one distance. After getting comfortable, it was just really feel for me. 

I agree 100% with this. Practice is the only thing that helped me determine.

Hello Kevin,

My tip would be too look up Butch,Go to 46 m point on the video.

https://www.youtube.com/watch?v=JtSwukkkZ8M

Forward or back in the stance will determine the distance providing the swing is the same.

I found this video  to be the easiest too understand how the bunker shot is supposed to be played.  

Sure, "practice" is the pat answer for -any- golf question, am I right?   But -what- to practice?  I think -that- is the nature of his inquiry guys.

For me it's a combination of ball position and how much I open my clubface, (along with which sand wedge I use in the first place).  The shorter the distance I need to cover, the farther up in my stance the ball goes, and the more open the face.  For the longest bunker shots, (pin across the green, etc), I'll almost square the face, with the ball back towards the middle of my stance.

The other side of it is, do I want to stop it with a lot of spin, or take a lot of sand and let it roll out?  That's just a matter of how steep or shallow I am coming into the ball. 

Hope this helps!

One swing thought I still use is trying to make the sand from the bunker travel as far as you want the ball to carry.  this generic tip takes into account different sand and club variables.  loft , bounce, angle of attack etc all have some influence on the ball / sand distance and trajectory.  its simple and effective  

It certainly is hard to judge, and this isn't a shot I really ever practice. For me, it just boils down to really picturing the shot that I want to hit and going for it (over simplified, I know). For instance, I had to hit out of a greenside bunker on Monday and just imagined how high I wanted the ball to go, where I wanted it to land, and how much sand I wanted to take with it. Choked down on the grip a little and hit the shot. Carried a little further than I wanted but not much. The most important thing with bunker shots is to always remember that you have to hit it harder than a stock shot.