Nick,
ah, you're probably right. In this case they are called 'spines'.
Washingtonias are all armed as far as I know and sure, some may have
smaller spines, but I don't recall ever seeing a spineless
specimen.
The difference is often obvious but if you're not sure just stick to the genus
and call it a 'Washingtonia'.
Washingtonias apparently hybridize easily so there are hybrids out there
like the ones planted at the SF Giants ballpark.
W. filifera is more cold tolerant than W. robusta. I included a
picture of one I pruned in Chico, an obvious W. filifera given
the location, stem size, petiole and leaflet size and color.
Wikipdedia has a good description.
'An Encyclopedia of Cultivated Palms' by Riff & Craft describes W. robusta
as taller, with a thinner stem and fewer hairlike fibers on the leaves and
leaf sheath. Leaves are deeper green in a smaller, more compact crown than
W. filifera.
Young W. robusta always have brown-red spines and W. filifera never
has a reddish-brown patch underneath the leaves near where the petiole
and the blade meet sometimes seen on W. robusta.