I have cheated a little and started my starter with a little bit of commercial dry yeast. I had some dry yeast from the last batch of bread that I baked. I added it to water. I left the water sit for a couple of days. Then I started adding flour to it, a couple of days later when I checked the jar it was full of bubbles on the top. I have added equal measures of water and flour to my 'starter'. But I am not sure if this starter is safe to use, considering that I cheated and started this with dry yeast.
How can I know if this is a safe starter to use?