Here's an easy thing to spot: just look in a grocer's shop or supermarket near you and you'll find foil-wrapped chocolate, crisps, football stickers...
If that's too boring (and it is), then you could look in a mirror: the backing of the glass is a bit of metal foil. The same principle is sometimes used to make jewels sparklier.
As it's Spot the Frippet day then you could always look out for the traditional pairing of the beautiful young lady with the plain friend. The friend is there to highlight the beauty of the other, in other words to be his or her foil.
A doubling-back sort of foil happened when a hunted animal retraces its steps in order to throw hunters off the scent.
A trefoil is used as a symbol by Girl Guides:
A quatrefoil is an architectural form much used in Gothic architecture:
Illustration by Anonmoos.
And a cinquefoil is a particularly annoying weed in my garden:
Apart from that, a foil is a sword with a safety button on the end.
Word To Use Today: foil. The hunting and baffling word (foiled again!) comes from the Old French fuler, to tread down. The thin sheet also comes from Old French, from foille, and before that from folia, the Latin for leaves.
No one knows where the word for sword comes from, but perhaps it's because any attempt to stab someone is foiled by the button on the end.