Pump it till it's firm.
It should make a tight 'ping' noise if you flick the leading edge, and if you try and fold the leading edge in half it should resist it but fold with a bit of pressure.
I've never used a gauge and never will. A gauge is only useful for getting consistent pressures (ie. pump it to the same number), not necessarily accurate pressure.

Best bet is to ask someone on the beach who's flying a similar kite if you can see how hard they pump it up. Even better, just offer to launch them and you can cop a sneaky feel.

I ride a Helix '09 and use exactly the advice given.

I flick it to get a nice ping, then try to fold, should resist but still fold.

i find if not enough air, hard to relaunch / wobbly, Too much and i'm scared it will explode in a crash.