Button in the HANDLE of the knife is key. Most spring assist knives have the nub or whatever on the blade. It actually does remove quite a bit of ambiguity of what is a "switchblade" versus spring assist.
I asked a MSGT on the OKC PD about the definition of a switch blade awhile back. The button on the handle, and the blade exiting the end of the knife constitutes a switch blade in his understanding of the law. Granted he is not Judge, jury, but that was his opinion.