On the surface this seems like a rather simple question, But imagine if the question was "What should I look for in hunting gun?" Essentially the same question, just a different weapon.
I found an article Now: Super Accurate Slingshot(Popular Mechanics Jan 1984) that talks about state of the art 30 years ago. Much like for guns there are entire web sites ...
The Pocket Shot!
If you want a slingshot for backpacking, then this is the one I'd recommend:
It looks unconventional, but I think it's genius, best part is you can store all your ammo in it and it packs away super small!
Watch this video: https://www.youtube.com/watch?v=6kY5ku1kE_M
You can even get a wisker cap for it and shoot arrows too:
In order to do this, you need the weight of the projectile and its velocity. The way to measure the velocity is to use a modern gun chronograph, you shoot the projectile through the to screens and it will measure the velocity for you.
Then you take the weight of the projectile and plug it into the equation,
(weight in grains * velocity in feet per second ...