Gauche（Scheme）で値の型を調べるにはclass-of #Scheme

classof.scm

(print (class-of 123))
(print (class-of "hoge"))
(print (class-of 'moge))
(print (class-of #t))
(print (class-of '()))
(print (class-of '(1 2)))
(print (class-of (cons 'a 'b)))
(print (class-of ''a))
(print (class-of (quote (quote a))))
(print (class-of (number->string 123)))

;; 結果
#<class <integer>>
#<class <string>>
#<class <symbol>>
#<class <boolean>>
#<class <null>>
#<class <pair>>
#<class <pair>>
#<class <pair>>
#<class <pair>>
#<class <string>>

Racketなどの他の処理系の場合はこちらに書いてある：
http://stackoverflow.com/questions/11566886/how-do-i-get-the-type-of-a-value-in-scheme