Phytoplankton

[BlueAbyss]

You could look at it under a microscope, do a quick survey of the species floating around in there and then decide if it's suitable. This will require a microscope, some time, a good reference (with pictures!) of phytoplankton, and a whole lotta geekiness. My sort of thing

Barring that (though a microscope would still be handy) I would say when it's a nice bright green and opaque, it's ready.