Iron Bacteria can be found in well water and other non-chlorinated water supplies. The bacteria eats the iron so it is not always possible to see it. Iron Bacteria found in well water usually has a habit of clumping in the corners of the storage tank on your commode.
So take the lid off of the storage tank on the back of your toilet and look on the bottom in the corners (if you can see through the water, if not, then flush it to check it). If you have sediment and/or iron and/or dirt or whatever, and it is smooth (or settled) across the bottom, no matter how much there is, with nothing "clumping" in the corners, it is most likely NOT Iron Bacteria. However, if one or more of the corners have something that looks like it is clumping or "building up" at all (not uniform with the other sediment and such in the tank bottom), it is almost certain that you DO have iron bacteria and will require chlorination. Any time you see anything clumpy, or slimy, or apparently growing inside that toilet storage tank, you should probably chlorinate the water in our opinion. You can also run your finger along the inside wall of the tank UNDERNEATH the waterline. If it feels slimy or slippery that is a strong sign of bacteria or organics. If it is alive, consider making it dead.... Then filter as necessary.