Programmatically check fixnum size

Wikibook for Common Lisp says that fixnums are guaranteed to be able to hold integer values in [-2¹⁵; 2¹⁵-1] range and that it is impl开发者_如何学Goementation dependent. *features* can tell me if the host is 32 or 64-bit but a fixnum in Lisp does not cover the whole 32/64 bits. Is there a way to programmatically check what is the size of a fixnum in a CL implementation?

(integer-length most-negative-fixnum) will tell you.