If you collect rain water just wait about 5 to ten minutes before collecting from a down spout so it can clean any dirt off of the roof and gutters. then what I would do is put a pond filter its like cotton you get it at the pet shop for like 6 dollars and then get a small container of activated carbon make a filter to fit in the downspout. You will want to do this at the top of the downspout in the gutter if possible so it will not clog the spout and you can manually clear anything stuck to the filter. Now your water should be clear if it is not you did not allow enough rain to clean the roof.
Now with a tds meter check the ppm of the collected water it will probably be about 10 to 30 ppm then check with a pH meter or drops the water should be 7.0 on the money if it is you can use it for your plants you will have to add cal-meg or magic-cal.I was tiered of haling water so I just bought a hydro-logic ro 100 my water is about 200 ppm from the tape when it comes out from the ro its 0 ppm good luck.