When you see new growth, that's proof positive the plants have rooted.

If you haven't watered already, I'd let rain be the only water until new growth.

I agree on leaving them alone. They don't need any water at this point anyway. If your curious, just pull one out of there and check it out. It looks like you have pachycereus marginatus there. I put two big ones in the yard, one with roots and one without. They both started growing at the same time and have put on the same amount of growth at the same rate. You will know when they are rooted.