Colombia's yellow license plates are the best quick clue you can hope for. Road signs also have the 'Colombian Cross' on the back - the metal sign structure looks like a big + sign.
Plates
Colombia uses yellow plates, which is unique for South American countries and a very useful clue. Taxis in Colombia commonly have their plate numbers printed in black text on a white background on the side of the car near the rear.
Road signs
Watch out for the Colombian Cross, and PARE stop signs.
Bollards
Poles
Occasional octagonal poles that look like Mexico's poles.
