A number of diapers (we are using pampers) advertise a wetness indicator. Does this simply indicate moisture or urine itself? I have heard people saying that this is useful for tracking wet diapers specifically but I don't know if it actually functions like that.
I believe the indicator simply reacts to moisture, not urine specifically.
If you're prepared to sacrifice a diaper, you could simply test it with water.
When my son was born, the hospital had Pampers diapers with a wetness indicator. When "off" it was invisible, and when the diaper was wet (used), a vertical blue line appeared on the outside of the diaper. This is a very clever way to show first-time parents that the diaper needs changing.
These diapers seem to be special hospital merchandise. I was not able to find them in stores, at least not here in Austria. I missed the indicator because it's so efficient, but we quickly learned to live without it anyway.