I found this out the hard way if the hole is empty with no solder in it like showen in your pic. If you hold the soldering iron on there for a little to long and try to put solder on it. it might look like there is nothing there take a quick look on the other side of the board the solder might be running through the board and pooling up on the other side of the board. My problem i was pre tinning the points and put on flux and put the solder down to the point a few times an noticed there was no solder left on my iron and it looked like there was no solder on the point i did this a few times then desided to look on the other side of the board sure enough there was a big bead of solder sitting on the other side of the hole.