Gary wrote:
The nest of plastic piping under our kitchen sink has developed an as
yet small leak. It's a combination of the main sink drain, half sink
drain and dishwasher drain.

What's the best way I can tell which of the many joints is leaking?
There's only dampness around the fittings at the moment so I can't
actually see anything dripping from anywhere.

Just dry everything off thoroughly with kitchen paper and monitor all
joints thoroughly and regularly until you find which one gets damp first.

David