Test if a value matches a constructor

At least get_pairs itself can be defined relatively simply by using a list comprehension to filter instead:

get_pairs xs = [x | x@Pair {} <- xs]

For a more general solution of matching constructors, you can use prisms from the lens package:

{-# LANGUAGE TemplateHaskell #-}

import Control.Lens
import Control.Lens.Extras (is)

data NumCol = Empty |
              Single Int |
              Pair Int Int |
              Lots [Int]

-- Uses Template Haskell to create the Prisms _Empty, _Single, _Pair and _Lots
-- corresponding to your constructors
makePrisms ''NumCol

get_pairs :: [NumCol] -> [NumCol]
get_pairs = filter (is _Pair)