This might help a bit:
http://www.orchidboard.com/community...endrobium.html
But if you want a general rule of thumb (at least for the phals commonly found in supermarkets and BB stores), phal type dens are ones that have blooms that ressemble phal blooms, and the spike comes out the top of the cane. The leaves are persistent. The Nobile types are the ones that need a winter rest. Those have short spikes that come out all along the canes and are deciduous. But if you want to ID winter rest needs of all the other Dens (species and hybrids), you need to know in which of the 6 groups it falls.