How can I tell if they're taking drugs?

The most valuable weapon you can have against drugs is a good relationship with your child.
And assuming they're taking drugs is NOT a good way of developing that relationship.

If only I’d done something sooner!

If your child believes you have faith in them, they're more likely to share details of their social lives. You’ll have a better idea of what they get up to , and they’ll be more likely to see your viewpoint, and appreciate your interest in their well-being.
What sorts of things should I be aware of?
Spoons discoloured from heating, tin foil, pill boxes, tiny bits of clear food wrap, sugar lumps, syringes or needles, cigarette papers, lighters, cigarette ends made of card, butane gas canisters, shredded cigarettes, pipes, small stickers or transfers, small bottles.
Money going missing; your child selling CDs and computer games, and you don’t know where the money's going.
These could be signs of drug use (but they might not be).
So what do I do if I find these things?