Concerning Harold's point regarding viewing traps in software, recent
versions of InDesign and Illustrator have an "Overprint Preview"
command under the View menu. Select it and zoom in: and all traps will
become visible. Look for dark colours at the edges where colours meet.
Traps made in these programs are typically overprinting strokes, but
there is a trap function too. Group the objects, then it is under
Pathfinder / Trap.
If colours are process, traps are frequently negligible, owing to the
"Common Colour Bridge": for example, where a blue meets a green, there
is a common bridge of cyan, so white paper will not show through even
if there is no trap, cyan providing the bridge.