Is there a type predicate that distinguishes pairs from non-pairs?

Yes. The pair? procedure is a predicate of arity 1. It returns the true Boolean value if given a pair, the false Boolean value if given anything else:

> (pair? 5)
#f
> (pair? #t)
#f
> (pair? (lambda (n) (+ n 1)))
#f
> (pair? (cons 1 2))
#t
> (pair? (cons #t (lambda (n) (+ n 1))))
#t
> (pair? (cons (cons 1 2) (cons 3 4)))
#t

This is an error and your program will crash.

> (pair?)

ERROR: Wrong number of args to #<primitive-procedure pair?>
; in expression: (... pair?)
; in top level environment.

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http://www.math.grin.edu/~stone/scheme-web/pair-ques.html

Copyright 1995 by John David Stone (stone@math.grin.edu)