In your question you suggest talcum or chalk, and this is what I use under sinks, in cars etc..
Kid's chalk comes in a variety of colours, which is handy if you need to trace against a white background. You can dust it on or draw it on in horizontal lines.
Sometimes I'll wrap or tape on a piece of tissue or kitchen towel, when I don't have the above powders at hand or when it's difficult to blow the powder on with proper aim such as deep under a sink, or all around a piece of pipe.
I also use soot dust (from burning some paper or a scented burn stick from my spouse's yoga department) or graphite from a dispenser in areas where I need strong contrast against white, of if I have neither of the above at hand.
For a long lasting mark you could use drywall setting compound, which would mark the water trail and then harden for inspection much later. I haven't needed this, but always thought this trick could be of use somewhere. Possibly cement or grout could be used too.
The benefit of talcum or chalk, and to some degree soot, is that they can be used in small amounts without too much worry about toxicity and lung irritation. Uncured drywall, cement and grout powder are corrosive and the dusting should not be breathed in.
