While things can be inspected and disassembled by their name with slime-inspect (C-c I) and slime-disassemble-symbol (C-c M-d), it’s often easier to do that through their definitions.
slime-inspect-definition inspects the object defined by the definition at point (e.g. defun or defpackage), works on functions, variables, macros, packages, classes.
slime-disassemble-definition disassemble the object defined by the definition at point, works on functions and macros.
Neither is bound to anything by default, but I bind them to C-c M-i and C-c M-d respectively.
One limitation is that they don’t work properly on DEFMETHOD, instead of finding a particular method they just use a generic function. I plan on fixing that someday.