I couldn't tell either for the first week I was trying to target feed mine....no polyps' tentacles were closing...just couldn't tell.
Then I got out my DSLR camera with a 50mm macro lens, a tripod, zoomed in 7x-10x, focused in on a polyp and watched on the live view feature on the LCD monitor on the camera.
Even though the tentacles didn't close, I was able to watch a smashed cyclopeez be eaten as it was moved by tiny fluctuations (almost like mini-muscle contrations but very subtle movements) closer to the mouth in the center, then S-L-O-W-L-Y ingested.
Later as the coral became accustomed to being target fed, the polyps' tentacles closed around the food. The other polyps will sometimes bend toward the polyps who have a hold of food as if to share. It's really neat to see.
I target feed 3x/week with smashed cyclopeez (and their juices) mixed with a small amount of shaved shrimp, Reef Roids, Selcon and phyto-feast. Sometimes I substitute super finely chopped mysid shrimp for the uncooked table shrimp. I would add DT's oyster eggs to the mix if my LFS carried it (and shipping wasn't a fortune now days).
Hope this helps
