What's a good rule of thumb to follow, to know how wet I should keep the soil?

I have proven terrible at keeping potted plants alive. I seem to always water them too much or too little. I'm currently trying to grow some cat grass for my cats to munch on, thought I'm keeping the two pots out of their reach for now; I want the roots to reach deep, or I know the cats will just rip them out, and I'll have to start all over.

The folks who sold me the soil, pots, and seeds told me to only water once a week or so. But the soil seems to get quite dry. In general, how should the soil look and feel? When is it too dry, and when is it too wet?