Here is what I have gone by. It is a quote from an article I found on line by Michael Connor.
"The luteola form has thinner but more numerous striations on the carapacial scutes (The second pleural scute bears 11-14 radiations in luteola compared to 5-8 in ornata)."
pleural scute = costal scute
So if it is just a matter of counting lines/striations it should be fairly easy, unless it's an older luteola and the lines aren't visible any longer.
I would imagine that there are some exceptions to this but believe for the most part it holds true.