Naturally colored Tahitian pearls only come in sizes of 10mm and up, so if your pearl is any smaller than that right away you know it’s probably a treated freshwater or akoya pearl. For larger pearls, you can look at the areas where dye will be most concentrated: around the drill hole, and any blemishes on the pearl’s surface. You’ll be able to see where the color has collected and is more intense. However, color treatments on gems of all kinds are getting better and better all the time, so the only way to know for certain that your pearl is naturally colored is with documentation from a reputable jeweler (if they’re not willing to supply it, that’s a pretty good indication too), or to send it to a gemological laboratory to be tested.