Polycarbonates are hard plastics, often used to make water bottles; these plastics are sometimes manufactured with bisphenol A or BPA. This chemical leaches, especially when exposed to heat or acidic liquids such as fruit juice or coffee, into the liquid inside the bottle. BPA mimics estrogen in the body and interferes with normal developmental, neural, and reproductive functioning.
Look at the printed information that came with the bottle. If it is not made from a BPA-leaching plastic, it should be marked "BPA Free." Also look for words like polycarbonate, lexan or polysulfone, as these are plastics that often contain and leach BPA. If the bottle didn't come with printed information, find out what brand it is and call the manufacturer to inquire about BPA in your particular product.
Turn the bottle over and look at the recycling symbol. Plastics that carry the number 7 are made with polycarbonates, which often contain BPA. Bottles that are BPA-free are likely to carry the recycling numbers 1, 2, 4 or 5.
Look at the color of your bottle. Colored plastics are usually BPA-free, as are ones that a bit flexible instead of rock-hard. The most problematic plastics are very rigid and completely clear.