How find out if bathroom sink drains in gray or black tank

Bobbo,

That's exactly the reason why I want to know... I'm not loosing sleep over that; however, I currently fill up my black tank by holding the flush hand lever for a few minutes (it's not a foot pedal). I like to have a fairly full (3/4) black tank when I drive to the dumping station, to help flush most of the solids. By filling up the black tank with the bathroom sink, that will save me some work and time.

Thanks all for your suggestions!