Both Dendros and Sun Polyps are referred to as Sun Polyps. Dendros are
Dendrophyllia sp, and Sun Polyps are
Tubastrea sp. However, according to Bob Fenner on the Wet Web Media; "
Balanophyllia, Dendrophyllia, not distinguishable from very similar
Tubastrea without examination of dead skeletal (septal fusion) characteristics." and "
Genus Dendrophyllia: Near impossible to distinguish from Tubastrea w/o microscopic analysis of corallite skeletons." All three of the above species are from the genus
Dendrophylliidae, there are several other species in the genus as well.
Generally, I think you just pay the exorbitant amount of money if the coral in question has large polyps and tends to stay open during the day, which seems to be the only easily definable difference, and is apparently not particularly accurate.
I bet there are more than a few people out there who paid for a Denro, but got a Tubastrea!!