(STRUCTURE-SLOT <object> '<slot-name>).

There is currently no portable, built-in way to access structure slots
given only the name.  If your Common Lisp includes an implementation
of CLOS that supports the meta-object protocol specified in the
original X3J13 draft spec (document X3J13/88-003), then it probably will
allow (SLOT-VALUE <object> '<slot-name>); however, not all
implementations of CLOS currently provide this.  Lacking this, some
implementations may provide implementation-dependent functions that
allow access to structure slots by name; note that this may cause
saved images to be larger, as some implementations normally open-code
structure accessors and discard slot name information.

While it is not possible to write a fully general STRUCTURE-SLOT function,
it is not very difficult to write version that handles specific structure
types.  For instance, after defining:

   (defstruct spaceship name captain position velocity)

one may then define:

   (defun spaceship-slot (spaceship slot-name)
     (ecase slot-name
       (name (spaceship-name spaceship))
       (captain (spaceship-captain spaceship))
       (position (spaceship-position spaceship))
       (velocity (spaceship-velocity spaceship))))

or using CLOS (generic functions):

(defgeneric spaceship-slot (spaceship slot-name)
  (:method ((x spaceship) (slot (eql :name)))
    (spaceship-name x))
  (:method ((x spaceship) (slot (eql :captain)))
    (spaceship-captain x))
  (:method ((x spaceship) (slot (eql :position)))
    (spaceship-position x))
  (:method ((x spaceship) (slot (eql :velocity)))
    (spaceship-velocity x)))

Another popular way to define this is:

   (defun spaceship-slot (spaceship slot-name)
     (funcall (symbol-function
                (find-symbol (format nil "SPACESHIP-~A" slot-name)
                             #.(package-name *package*)))
              spaceship))

I personally recommend the first version.  It is likely to be much faster
and more memory efficient than the second version.  It's also easy to get
the second one wrong; many people forget to specify the package argument to
FIND-SYMBOL, which can cause incorrect results when the package at run time
is different from the one at compile time.  Even my version assumes that
SPACESHIP-SLOT is being defined in a file that is in the same package as
the one containing the structure definition; if this isn't the case,
#.(PACKAGE-NAME *PACKAGE*) should be replaced by a string naming the
correct package.

Another workaround is to define a MY-DEFSTRUCT macro that parses the
defstruct arguments and expands into a call to DEFSTRUCT along with a
definition of the runtime slot-accessor function.

Some non-portable techniques include the use of SYSTEM:STRUCTURE-REF
in Lucid (LCL:STRUCTURE-REF in earlier versions of Lucid) and 
EXCL:STRUCTURE-REF in Allegro.