There's no consistent way round, it is how the disc is manufactured as a unit that determines whether they're backwards or not. Both the grooves and the vents work together to deliver the best braking and cooling ability that the disc can deliver. I did think they were meant to 'sweep' inside to outside, but that theory was disposed of when I went to see the BTCC a few years ago and saw their discs apparently on backwards!!
EBC brakes (from what I remember) sweep from inside to outside, and the vents are also designed to work the same way round. But I remember an old manufacturer who made black discs and they were meant to mount 'backwards'.
Dyscontinued, your bolt alignment can't have been caused by the orientation of the disc. Every wheel has equal spacing between their bolts, whether they're left or right. If your discs had a 4 bolt and a 5 bolt pattern on them (as alot of discs do, like the left one of ibizacupra's picture), you might find rotating them a hole would have given you your fifth hole.
James.l, check with the disc manufacturers before you go to the effort of swapping them?