You can't. There's no way to tell if someone is a pickpocket or not by looking at them, and there's no way to tell if pickpockets are around until after your pocket has been picked.
What you can do, though, is make life difficult for any pickpocket who might want to steal from you. Don't carry money or other valuables in a back pocket. Use your front pockets for things you need ready access to, like your cash for the day. For stuff that you want to keep on your person but don't need to pull out often, keep it under a couple layers of clothing. Get something like this:
and put your passport, tickets, bank cards, etc in it. Wear it under your shirt, next to your skin. Pickpockets will have a hard time getting it from you without you noticing.