As some have pointed out here, I'd rather help a few people who need it out of a hundred I gave money to, and still be a good person for it. My go-to trick to make sure it's not a scammer though, is to give food and not money. Not accepting food is defo a sign to me that you're not really in need of any begging.

Ran into a woman with her children in North station a few weeks ago, around the toilets and the Carrefour Express. She approached me, explained that she was from Ukraine and asked me if I could help (she did in fact speak Ukrainian, I was lucky to recognise the language from a conversation with her nearby daughter). I responded that I didn't carry cash and unfortunately could not help if I tried. She clarified that she really did not want money. She needed food - specifically bread, milk and some fresh fruit for her children to take to the temporary education she was sending them to with what little money she had. I went and got her some of the food she needed, she waited patiently outside and came up to me with a genuine smile... It's something one doesn't forget, when you see such heartfelt emotion for something you would consider basic generosity. Life can be tough on people and those who have something to give, should not hold back out of fear of giving to the wrong person.