Well, you can solve half the problem with just one of the simple gauges that fits onto your pump. We sell a couple of these (one from Slingshot, one from PKS) and they're both only $20. I used to only recommend them to beginners, but I started noticing that when I was landing/launching kites for proficient riders that their kites were under-inflated. Now, I recommend them to everyone. It just takes the guesswork out. And, if you KNOW that you pumped it to 8 psi and check it later b/c it feels *squishy* and discover it's less, you may have identified a leak before you find out the hard way.
I always tell my students, "Remember, limp is NEVER good!"
I do admit, it would be cool if they came up with a monitoring cap-gauge, but I have also heard those tend to leak as well.