The only way you'll know which of your friends are on the Orange network is to ask them! Because it's now possible to retain your number when you switch between companies, you can never be sure (from the number alone) which network the phone is on.
For what it's worth, here's a list of all the mobile codes which are allocated to Orange (but, as I've said, someone could start with Orange and transfer the number to another company):
07773
07779
07790
07791
07792
07800
07811
07812
07813
07814
07815
07816
07817
07855
07866
07870
07890
07929
07966
07967
07968
07970
07971
07973
07974
07976
07977
07980
07989
Chris