How do you know if your a real mermaid?

usully when they sing, it sort of puts people under a "trance", you would say, although age- old mythology says they're always near water/ in water, etc., however, modern-day mermaids try to avoid water at all costs to avoid showing their iconic true form, the mermaid. the types of mermaids i'm talking about here is sort of the "convertable" sea-land mermaid, which is much like a daywalker vampire. (except that when this type of mermaid comes in contact with water, s/he will begin to change from thier human form to the mermaid, which is why they try to avoid any situation that involves water at ALL COSTS)