Water detection capsules are of more use for JET A than avgas as they are for detecting water in suspension. JET A is more likely to abscorb water than Avgas.
Water detection paste is better for avgas as you can check the botton of a container where water will lie.
Out of interest the CAA flight safety magazine Clued Up dropped on to the doormat this morning.
page 62 has a discussion on testing fuel and storage.