If someone is actually trying to run a scam, don't be afraid to be rude. People who are afraid to be rude or say no will always be stomped on by con artists and salesmen. There is nothing wrong with being rude to them. If you worry about being wrong, then you can make up some reason why you can't do what they ask. If they ask to use your shower and say that they're at risk of failing a job interview, tell them that the water has just been turned off. You can also admit that nobody is allowed to see the inside of the house for security.
If a homeless person asks for a 20, offer 5. I've taken some awkward white boys into the city who think they're going to get stabbed if they don't turn over money when a homeless person asks. We're not going into a ghetto either. Certainly old and urban, but not a ghetto by any means.