You can figure out your bonus % when you collect from the Gulp N Blow. It normally pays out 100, so anything extra on top of that 100 is your bonus. For instance, if you collect 150 from the GNB, then your bonus is 50%.
Well, yes, but it's not the most accurate because the decimal will get rounded (up, I think). Instead, try the holiday tree. It gives $500 a day. Subtract $500 and divide the remainder by 5. For example, mine just output $572. So, my town is getting a 14.4% bonus.