Rain water safety

I know that urban rainwater is considered to not be potable, but I did consider whether to use it on my few vegetable plants.  I decided not to use it on my tomatoes and other veggies.

Dust and ash (from incinerators and coal power plants) are one source of contamination.  During a dry period, windblown dust accumulates on roofs.  Ash accumulates year-round, depending upon wind direction from the incinerator or power plant.  Minerals and toxins from those can be reduced by not collecting the first rain after winter or after a dry spell.

However, bird droppings and squirrel droppings can't be avoided so easily.  I could test for minerals and toxins to learn what my water is like, but bird droppings are a more random source and I never know when a flock of something will pass by.

I'm not yet interested in adding filters suitable for removing bacterial contamination, so I'll use potable water for the vegetables.  I don't have many of those anyway.  The flowers are thirstier, so they'll get the rainwater.