Not a bad idea, actually, if you use different colors for different places!
The window leaks on these cars are insidious -- they only leak around the seal to body area, only on the bottom edge (unless they have been glued in), and usually only in the lower corners were the wires for the electric defroster grid go. The water from the back window will leak under the seal and under the sound deadening on the back shelf -- the shelf cover won't get wet most of the time. The water runs under the sound deadening (molded ashphalt sheeting, actually), and slowly works its way down the seat back and out onto the floor, unless the 123 chassis is radically different than the 115.
The usuall "tattletails" are small water marks on the gasket on the bottom edge, and water trails in the trunk.
Unfortunately, the floors slope back towards the rear on these cars, so a leak up front can cause wet rear floors.
A leaking windshield up front will usually drip water down the kick panels or off the firewall under the carpet -- you will need to get the carpet and pads out to check for water trails.
Water leaks are a true bummer -- takes forever to find them! See previous post for checking the condition of the cowl/sunroof drains, too, and check for torn rubber drain hoses in the trunk.
And just for fun, check the rear door gaskets, too, in case one is torn and water is leaking down the door -- not likely, but hey, I'm out of ideas other than the window seals!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!