Most times you'll see the spines have worn down. But if you look closely at the picture, you can see the real spiny ones. Sizes range from 3/4" to 1-1/2" with 1" being the average.
The good news is these shells are found on most Southwest Florida beaches. After a while you develop an "eye" for them and soon you'll be able to pick them out from the many varieties of other shells laying on the beach.