Four-leaved clovers are supposed to be lucky, but the three-leaved varieties are very beautiful, too:
There are lots of different plants with trefoil leaves, many of them members of the pea family, so I suppose that if there was no other way to spot a trefoil then you could always plant a pea.
Or look out for a Girl Guide:
This is another trefoil symbol, but of something even more dangerous than a Girl Guide:
Spot the Frippet: trefoil. This word comes from the French trifoil, from the Latin trifolium, three-leaved herb, from tri, which means three, and folium, leaf.