Sun polyps (sun corals, tubestrea) are best fed once a day. Each polyp should be fed individually. I also don't know if there's anyu factualy information about polyps sharing nutrients, but best results have come from feeding each polyp individually.
As for knowing when it's open or not, the polyps will be completely extended like a bunch of fingers wide open waiting for something to hit them. When the poylps are closed, it looks like a tube where the edge of the tube is recessed into the rest of the tube. They have very distinctly different appearances. Like spline9 said, if you feed them on a strict schedule they will be "trained" to open their poylps at the same time every day, waiting for food.