A few years ago, whilst I was about to get in the train, a kid (about 15 year-old) asked me, "what time is it?". Since he used Englished in Paris (where people speak French), I felt a bit strange. So I shouted to my husband in Cantonese saying beware of pickpockets. The train was very crowed at that time, his partner tried to open my bag but I resisted. Everyone then knew what was happening and the two kids ran away. My hints are
(1) If someone asks you something in English, watch out.
(2) They normally have partners, one of them distracts your attention whilst the other would take away your wallet.
(3) They choose to do it at the time the train door is about to close so that they can escape after getting your wallet.
(4) Those big stations with lots of exists and connections are extremely dangerous.
(5) They usually start work after lunch time, so morning is more safe.
(6) Don's use wallet at all, use those belt which you can wear it between underpants and trousers. You put your cash and credit card inside the belt (they are available in luggage store).
(7) Ask hotel or restaurant to call taxi for you. Some taxi are fake.
Beware of kids. They normally work in pairs and run around you in the metro. I have once encountered a pair of girls running around my wife in the metro near Sacred Heart, who tried to divert her attention to open her purse.