How nice is too nice?

Ever hear that story about the angels here on earth? It says something to the effect of Angels in heaven being really busy this time of year, so God places human angels around to do the work unknowingly. A doctor starts having car trouble on a trip... pulls into a gas station where a woman just started crying because she just spent her last $5 on gas... but she's trying to get her & her kids across country.. so the Dr. fills up her tank, buys her food from McDonalds & sends her on her way. Of course nothing was wrong w/ his car afterall....
How do we know when we have been deemed an Angel to help someone else out? How do you know for sure when an angel is helping you... or is this someone that wants something in return? At what point do you know it's ok to trust someone new in your life?
I have met a new friend...and I feel he's really being tested by God. He is a single father, caring for precious 2 year old girl. At almost every turn he makes in his life something is going wrong. There's not much I can do to help. I don't have money to give him... but I've given some clothes for his daughter. I can't give him a better job, but I've shared meals w/him & his daughter. I just wish there was some magic button I could push to help him out...but then again I wish I could push that same button for myself & few others!